I'd seek input from Vlad, as he is now based out of Dutch John on Flaming Gorge, which is a pretty high elevation airport.
I am a contrarian here, and would probably rather have a 141 HP Turbocharged Rotax 915IS, that will deliver full horsepower, regardless of the elevation, as well as run on 91 octane premium unleaded Mogas, if you want to.
Sure, it won't be as peppy on take off as a O-320 near sea level, but it will be peppy above 7500 ft, elevation, you always will pretty much get the full 141 HP when you need it. Your needs are based on being out of Colorado, so that's why I think the turbocharged motor is the way to go, you can always get 141HP out of it, regardless of the elevation. At what point, density altitude and elevation wise, do you have 12% loss of atmosphere? Above that the turbo version wins, plus the motor will make your plane a feather weight, compared to a o-320 or 0-360, the Rotax weighs MUCH less.
At 3000 ft elev, you are at 900 hPa. That's 10%, at std conditions 15C and sea level. At 25C and 3000 ft, that's 4250ft DA, so you can see that the turbo model Rotax compares quite well.
Keep in mind, the 9A proof of concept was built with a o-235 at 118 HP.
Price, of course, would be a BIG factor, but when you need a turbo, you need a turbo, especially if flying around in the mountains.
If your friend wants a truly modern build, then, in keeping with that theme, he should go whole hog, and install a modern designed motor in it too, built from modern materials, designed and tested on a computer, and a clean sheet white paper, not something from the 1950's. The 915 IS Rotax is very new in materials and design.
The 915iS is doing just fine in the TAF Sling 4 seater TSI model. Might be worth checking out.
Flame suit on.