I love cooking up with rules for this kind of stuff, but for a One-shot it should focus more on the feeling and story than the actual rules. Westerns are about the law against the chaos and lawlessness of the world. No one is there to save the heroes if they fail. Clashing views and revenge plots aplenty. The Bad Guys can be simply selfish and evil, but it’s more interesting if they are sympathetic as well (The mayor hurt them so now they want revenge). It doesn’t need to be balanced, and if the question of balance comes up, just be sure to err on the side of the players.
This can be done in ICRPG, but @Jaide did well to recommend other systems.
If you wanted to run it in ICRPG, keep it loose on distances and ranges (Close, Medium, Far, REALLY Far)
Normal mobs can be run in big singular monster entities:
1 
+5 to rolls (reduced to +2 when mob is first hit)
1d8 gun damage (goes down to d6 when first hit)
Mobs can affect multiple targets in a zone
Mobs can only take 2 actions a turn
Mobs cannot benefit from COVER
Or you can run individually as 1HP enemies that go down after a successful hit (No damage roll needed). You can mix and match these versions as needed for GM versatility OR pick one to reduce cognitive load.
For players, instead of normal ACTION/MOVE, you could do a 3-Bullet turn (very much like Pathfinder’s 3 Action Points Turn) where the players can do an Action, Attack, Move, etc., using one of their Bullets per round. This method allows the user to just go all out and do 3 attacks in one turn, or move move shoot, or move DON’T DIE ON ME, MAN and shoot.
A Meta-Currency you can use instead of the Hero Die can be GRIT (3 maximum). These can be used OUTSIDE of their normal turn order to assist each other (Give ADVANTAGE to attempt roll) and help protect (Reduce the damage of your ally by 1d8; and explain how your quick thinkin’ saved their hide!) or finish off as needed (Max Damage on the assisted attack; If enemy is not down’d, they must SAVE or fall down dying).
Now we introduce The Devil’s Gamble mechanic. Get a deck of playing cards and then play a simple game of HIGH OR LOW with the player. This activates when players use GRIT. If the player wins, the action only requires the one GRIT. If the player loses, then the action requires TWO GRIT!!! Pay the Devil up in either Scenario. If you can’t pay the GRIT, the player character can either choose to have their action fail OR succeed with a cost!
Another Classic and very traditional Western Shoot Out trope is COVER! Keep it simple by just having 3 tiers of cover and they just give Temp HP. The tiers can be as simple as 5, 10, 15 HP. If they move away from the cover, then they lose that Temp HP. Cover does not add defense or AC.
