Honestly, I always wished there were D&D based SF/Wester/Whatever rules back in the day. I know Gamma World was kinda close, and the DMG had Boot Hill conversions.
The questionable thing though, is do you really need a whole RPG to do it?
Just come up with classes and stats for weapons (and maybe skills), and boom, you're done.
In my retro-clone that I entered in that ENWorld RPG contest (got 4 votes!), I came up with Star Trek style classes readily enough, and I cribbed the skills from Traveller