Re: Propane conversion
That is why I planned to just run the fuel valve for 90% power (about 30mph) and let it waste or bleed out the air filter during idle/heavy throttling. A 50cc 1 cylinder engine should be able to run just fine off it.
I've seen cars with the complicated metering systems, and in country towns I've seen some gasoline trucks that ran on propane, via lines run into the filter box. Pretty much the low-tech trucks were made to start on gasoline and accelerate to highway speed, then run on propane via a switch for cruising.
re: boost. I don't know if it would do anything to a small kit bike like a happytime, but propane injection has been used reliably for heavy-haul diesel trucks and construction equipment.