Now as far as making your game have the equivalent of levels, that’s where things get a bit tricky. If that is how you want to run the game then I would actually recommend running a game system the supports level play. There are ways to make games like 5e play more like ICRPG. 5e Hard mode being one of them, you can find that on DriveThruRPG, or even better from the source its self Runehammer.
For my self and my game I am implementing an old White Wolf game method of gaining and spending experience points. Where players gain at least 1 Exp for showing up then addition points if they learned anything new about the game, their characters, or the movements and motivations of their foes. Additional points can be accumulated like hero coins, and Unspent Hero coins from an evening net the character +1 exp but they start the next session without the hero coin. Similar to Mastery in Master Edition every 20 natural 20s earn the character more exp to spend on advancement, I have it written in my notes as +3exp but that is subject to change.
Spending the exp as above work like this, the players may spend any amount of EXP at a place of rest (like a town, monastery, guild hall, etc. basically any place they can train or learn new abilities, spells, or acquire new items)
1 XP - Random Shabby Loot
1 XP - 100 Coins
2 XP - 1 Chest Reroll
3 XP - Random Setting Loot
4 XP - Random Epic Loot
5 XP - +1 Stat Bonus
5 XP - Generic Milestone Ability
5 XP - Favored Type Milestone Ability [the first one you start is your favored]
7XP - Non Favored Type Milestone Ability
7 XP - Favored Milestone Path Reward [the first one you start is your favored]
8 XP - Heartstone Piece (1of 3) [collect 3 gain +1 heart]
10 XP - Non Favored Path Milestone Ability
As a side note I still may offer a unique milestone reward for excellent play or specific goals being achieved, for example if a player is playing Rothgar the Barbarian seeking the War Maul of Chieftains and follows all the hooks and clues to find its resting place then the character acquires the War Maul of Chieftains. But to further empower the once fable relic to its full glory the character may need to perform a ritual to cleanse it of the previous chief’s soul or some such.