I can't recommend Sidewinder: Recoiled (Dog House Rules) for Savage Worlds enough. There was also a d20 Modern version back in the day,Agreed on the lack of mostly-historic Westerns.
There are several on DTRPG that I've not looked at:
Cowboy World (PBTA, has a chapter on "Going Weird"
Colts & Crooks (the description section is a poem...)
Sidewinder (setting book for Savage Worlds)
Rider (I suspect it's a Traveller/Cepheus retheme, as it's from Independence game.)
Cowpunchers Reloaded (doesn't specify system)
I will add
GURPS Old West: Revised Edition (Steve Jackson Game): a GURPS historical genre sourcebook for GURPS 3e
Western Hero (Hero Games): a genre sourcebook for Hero System 4e. I think there may be an updated version for the current edition of Hero system.
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