So basically, one would need to thoroughly go over every single monster in those modules and convert them on one to one basis ?
That sounds like not only lot of work, but also something you really need to have knack for game balancing to do…
I was thinking of another solution though. Circumventing this, by simply let players gain HP on level up.
I know it goes against ICRPG, but its easy way to fix this…
Unless someone has idea, that does not involve modifying each monster on one to one basis.
Converting monsters is so easy
Giant lizard
2 attacks, 1 heart
Bite: basic effort, close
Tail swing: weapon effort, close, knock back
I just made that up completely off the top of my head with only 2 games experience with ICRPG. And that was converting a lizard from real life. 
You are definitely over thinking this.
don’t forget, you don’t have to convert all the monsters all at once, you only need to convert monsters used in your upcoming session. plus by only doing a little at a time you’ll have the lessons learned from the previous session to be able to adjust. remember to keep it easy, it’s supposed to be fun not work.
I forget where I read it (I did try to search here, and on r/icrpg), but someone somewhere simply said “don’t worry about converting”. Just lower the HP on the monsters. Monsters can have increasingly dangerous attacks as the campaign advances and new baddies show up; but not all monsters need to have tons of hit points.
Having a lot of hit points can tend to lead to longer-than-necessary fights. Keep most monsters at one or two hearts (10-20 HP), with a healthy complement of mooks (1-5 HP) that can be easily dispatched. Save the 3+ heart monsters for your showcase baddies.
Leave most of the monster attacks as is. Monsters are dangerous. Getting hit and hurt adds tension to the game, and encourages players to keep things moving. It also helps them think about using their loot in fun and creative ways.
There’s also this similarly-related video from Hank about dropping Pathfinder bits directly into D&D 5E, without rebalancing anything:
We generally discovered that if a monster has two hit dice, that’s roughly two hearts or 20 hit points. If it has 3 hit dice, that’s three hearts or 30 hit points. It’s not perfect, but it’s an EASY way of converting monsters quickly. The same goes for their bonus to hit. +1, +2, +3, etc., depending on their hit dice. Two hit dice equate to a +2 bonus in their main stats or all their stats. Keep it simple.
Then, just grab one or two special abilities that are unique to that monster. I’d do the conversion session to session, using just the monsters/enemies I need for the night — no more than 5 or 6, generally.
So, gnoll, 20 HP, +2 to all attacks, has a power attack that can do 1d8 damage. Done.
Owlbear, 50 HP, +5 bite and claw, grapples with contested +5 STR roll.
That took me five minutes.
Well you tell me what are the 3 main aspects of an owlbear and a gnoll?
I don’t know much about either and I don’t have a monster manual beside me atm.
That said I’ve heard owlbears are created and not natural. So probably no many of them around, solo monster battle
2 hearts, 1 attack, +3 on all rolls.
Sharp claws attack: weapon effort
Screech: targets con save against stunned for one round.
So is that good I dunno, it’s late. But it’s easy to make up straight from the fiction,
Though of course it could have more d&d flavour if I had a source book in from of me.
As others have said you don’t need to convert every monster just the ones you need for that session. So what maybe 4 at most.
A few additional things to consider:
Weak monsters can simply be 1 HP monsters. If I were to convert some gobs or kobolds this would be the first thing I would do. Example: Goblin: 1HP STAT -1 Rusty Sword (D6). DONE
The effort dice make converting damage immensely easy. Does the monster make claw, tail, fist attacks? D4! Does the monster carry a weapon? D6! Does the monster shoot fireballs? D8! Super simple.
Having a single stat value for the monsters is really quick and straightforward. Weak monster: Stat -3 Strong Monster: Stat +3 (for example).
In my opinion one of the great things about ICRPG is that you don’t have the boring and mechanical ramping up that you find in D&D where everything increases in a relative manner. It’s all about all the cool and unique LOOT that makes “leveling up” so much more enjoyable. Why not just assign monsters some LOOT instead of having to come up with special powers or abilities…or just snag the LOOT abilities and use them as the special powers?
I have to add that ICRPG allows creating monsters that are not standard fairly easy. I have lived through OSR before it was OSR and I can say that using the predictable monster stats gets a bit dull and monotonous. I know you are trying to convert the OSR monster stats to match a module but I would encourage you to not get caught up on perfect translation, this is your chance to be creative and throw some interesting quirks in there…your players will likely appreciate it!
Stats are really easy to adapt (there’s a bit in CORE book about making your own monsters). You can quickly get a feel for this.
Eg:
Weak (zombie/peasant) = 1 or 5HP, +1 rolls
Normal (guard) = 10HP, +2 rolls
Strong (knight) = 20HP, +4 rolls
Huge (ogre) = 40HP, +8 STR rolls
Improvise or take and adapt abilities that suit the monster.
Eg. owl bear:
Claw attack: on hit, contested grapple check
Beak attack: does ultimate dmg if grappled
Screech: con save or stunned
Very “healthy” - 20-30HP
2 Actions if solo, or simply more owl bears, or Enrage (if under 50%hp, 2 actions)
(Stick to 2-4 actions/spells per monster or you just get overwhelmed and PCs can’t “learn” them)
I don’t do hit points for my monsters, I do Hits. Dungeon Craft turned me into this method. The monster takes 1 hit, 3 hits, 6 hits, or for a bbeg 9 hits.
And basically I have a number of “monster slots” per encounter per PC. 2-4 1 hit monsters full one slot, 1 3 hit monster fills one slot, one 6 hit monster fills 2 slots. Their basic damage is a d6 plus whatever their Hits are. A 3 hit monster does d6+3 as their base attack.
Throw the idea of balance out the window, and your tabletop gaming experience will evolve dramatically…literally.
This is the way to do multiple creatures with little effort. No question. It also takes effort off the table, and the DPS race becomes nullified. However area attacks becomes a bit overpowering if your players game the system.
I would keep HP with less than 5 creatures. But use hits with 10 or more. The middle ground is up to the mood and markers available.
Though, if a player is using his big bad vorprol hand grenade of unobtanium…they might roll a d4 or d3 for number of wounds. Or I just clear the section of foes where it hits.
As to the creation of creatures and translations.
You could do it, but it’s an effort.
You know the +s a creature has. You know the attacks they have.
Quick notes, and it’s done.
Or use them as is. They do translate pretty evenly with a bit of flexibility.
But HP needs to be changed, long combats are just that. Long. Not interesting, not dramatic, but a slog.
Sounds great. But what about players investing points in extra damage ? On their Melee effort or magic effort stats
If their sum is significant enough I’ll count it as two hits. Truly, though, the difference between several hit points has no real effect in game play. Say you go for a 3 round fight in which a players does 3, 6, and 4 points of damage totaling 13. Did tracking those individual numbers really add anything to the game? The players will see no difference between 3 hits totaling 13 HP and you just tracking 3 hits.
I agree with you. But would my player agree, after he invested 2 points into melee damage bonus ?
We will need to remove that option completely. ( fine by me though )
Effort still exists. But it’s not really significant for combat.
Opening the lock, perception, pushing the boulder, hold the door!!!, social encounters…
Or go with a hybrid method. Boss fights to 20hp~30hp at TN 18 are much more dramatic than 6 wounds. Specially when recovery, life steal is involved, health potions are involved.
DPS or DPR (damage per round) looses its significance, but the hard alpha strike (hit them with your most powerful strike first) capabilities don’t.
This also presents fun capabilities like spell scrolls and other consumables that can do damage to large groups of foes…but not work so well during boss fights.
You can say that 4 hits translates to 4D6 or that it’s not meant to be used on HP monsters. I have not really seen what people would do in those situations, since I use hits/mook rules for large encounters.
However, don’t mix one with the other during the same combat.
I hear ya. Players like to see bonuses.
But A 3 Hit monster is basically a 10 HP monster. And At the end of a battle, though, it all events out. Rolled a 2, rolled a 6, rolled a 1, whatever. Add your modifier. It’s still not that different or outcome changing tracking HPs vs Hits.
In the end, it comes down to the narrative effect a Hit has.
Do players know how many HP the monster has anyway?
Many players take the average for their HP increase when bumping up levels. DMs do average monster damage rather than roll.
Whichever method, it all averages out. With that view, Players will want to put Points towards consistency rather than spike. It’s all a numbers shuffle
I prefer this method because I’m running a group of 6 people brand new to RPGs. It speeds the game considerably.