Just pondering the possibilities. The small engine conversions for lawn mower sized engines seem to do well, burning extremely clean, both internally and exhaust. I’m mindful of how bulletproof the old 5000 lb forklift we had in the shop was years ago. Literally just gas and go for years at a time.
Has anyone played with propane or butane in a hit and miss? It would seem to me the fuel part is pretty easy, either by directly introducing propane into the mixer, or using a torch type Venturi mixer to inject fuel mix. The stumbling block would be some method of oiling the top end. I almost think that for as little oil as the top end needs, I might want to try some sort of drip oiler dripping directly onto the intake valve stem on the top of the vertical cylinder. There would appear to be plenty enough clearance to allow oil to wick down the valve stem into the intake.
I’m also daydreaming about running the in-line four I have ordered on propane or butane . That one is pressure lubed, no oil in the gas, so it’ll be easier.