Hello Dog Walker.
For the crank, you'd want either 3/4 or 5/8 diameter...with a key way notch.
Regarding mini bike specific, I've learned from
@markus and
@cfh there were mini bike specific engine models with specific attributes devoted to only mini bikes. The threads are very interesting. Perhaps they can point us to their threads.
And for just a rider, those China carbs are great (imo)...did one on an hs40 bird war eagle, one on a 6.5 predator boonie, and another on a 6.5 predator generator, all seemed plug and play.
Whereas I spent the extra money for an OEM carb for my Super Bronc 8 hp tecumseh.