Even if you do balance the mass perfectly for the piston / crankpin / rod & crankshaft...

you will still have vibration - especially in the sideways plane from the rod / crankpin & counterbalance.

To get even near perfection, you need 4 cylinders to start, 6 is better.

But back to basics, if your engine is really bad for a typical glow, then you can bolt a small flywheel onto the crank.

It must be secure & marked so it cannot slip. Then drill a little hole (taking some mass away) when piston is at TDC in the flywheel at 180 degrees.

Is the vibration any different....? Its trial & error - but will give you an idea of weather the crank needs MORE mass balance - or LESS.

You can get vibration measurements using accelerometers and a 'Pico' device linked to your PC.

Depends again on how far you want to go...

have fun!