Re: 50cc GY6 on a Bicycle??
My dad used to take shafts and weld them up to a larger diameter and then turn them down to desired size and then cut a keyway in it.
Yes a bit more effort but that would make for a dang good setup that would be easy to swap sprockets on, the shaft isn't hard to take out of that little gearbox, they actually make a couple different ratio gears that can be swapped out in them for eother higher or low final ratio, and the gears and shaft is actually purdy cheap.
the shaft is to small to put a keyway in I would think, but welded up and then turned down to 3/4" would probably be perfect.
I dont always ride WOT, but when I do..I do my best to keep the rubber side down..Ride safely my friends!