My 48cc engine's pinion sprocket was wobbling, I wanted to true it up, so I elected to remove it with the biggest, baddest slot head screwdriver I had and the pinion bolt's slot actually stripped out. Apparently you're supposed to heat up the pinion bolts now because they red Loctite them at the factory.
So I drilled the damn thing off, pulled the pinion and drilled a 1/4-20 threaded hole in what was left of the remainder of the pinion bolt crank (somewhat off center) and used an grade 8, 1/4-20 bolt and cupped washer to hold the darn pinion on after tapping it into near perfect alignment, well it's quieter and the gear teeth are looking pretty good, so far.
Otherwise the engine has been fairly reliable. It hasn't had any trouble getting started. I credit the Yamaha PW50 clone carb with it's better metering, and start enrichment circuit instead of choke with it's better running. I like that it's happy speed is about 24~27 mph. Seems to get fair gas mileage I'd expect over 100 mpg at the throttle openings I use and pedal assist, I'm getting about 70 mpg average. I don't think they'll ever get decent fuel mileage.