These arent spin balanced at the factory, I guarantee you.
The size of the flywheel holes should be a percentage of the weight of the top half of the connecting rod plus piston/rings weight plus wrist pin weight.
You can increase the hole size or you can use a lighter wrist pin. Both affect balancing in a positive way on this engine. But there is no getting around the need for a 2 stroke CDI to replace the 4 stroke one that comes with it because it has too advanced an ignition timing at high rpm.
Fabian, your non-understandable video (because of teachers heavy accent) doesnt prove what you are saying. But anyway your statement of 2 strokes not being able to be balanced is too broad. You need to give details and specify.
My position is that an engine w/o an expansion chamber can be balanced enough so that the handlebars don't vibrate excessively which is tolerable for bike riders seeing how they adjust themselves to so many discomforts anyway. The standard Grubee engine vibrates at an untolerable level. It can be brought closer towards perfection, enough to make it a pleasant experience to ride.