Thank you for the great idea mdlee1958, I'll keep an eye out for one of those motors. The YSR was pretty popular around here so CL usually has a few or parts. Team Calamari Racing makes a bunch of stuff for the YSR engine.
Of course the bicycle with a geared engine is technically a motor driven cycle and needs a motorcycle registration, at least in CA. I wonder if this also applies to running engine power through a derailleur or geared hub, as it could be argued that those were on a bike before motorization.
Internal gears are way stronger than a derailleur or geared hub setup, it would be nice to not worry about all the complication of shift kits and extra chains or bike hubs that might not hold up.
It sucks that if you want to be 100% compliant its difficult and expensive to have a multi-speed transmission. I wonder if semi-automatic transmissions still count as automatics under CA state law, that's how they are classified in other types of vehicle, why not in bikes? The ATV style semis in Yamaha PW80's or similar style engines might fit a legal loophole niche for motorized bikes that need good hill climbing without all the external complications.