Gy6 50cc
 

What’s the best way to tighten the flywheel bolt ?
Any suggestions on how to stop the motor from spinning
So I can get correct torque on it?

An impact gun works best, if you don't have one you can make a strap wrench out of some rope and make a half hitch around the flywheel and make a loop then use a big wrench or pipe or something to hold it, the more you tighten the tighter it gets. (also good for removing oil filters). Another way with small engines is to remove the spark plug, turn the engine until the piston is down in the cylinder then feed in a bunch of rope to fill the cylinder with it. That will lock the engine so you can tighten that nut. I prefer the impact or half hitch method.

Isn’t the impact method dangerous? Overtighten possibility.

Yeah, the impact isn't really the official factory approved way.... depending on the impact gun two or three uga duggas ought to work. If you really want to do it the proper way a flywheel holding tool and torque wrench is the way to go.

I’ll have to give it a go, hopefully one of the woodruff from my kit will fit