Engine vibrations are nuisance. So I am actually thinking of getting deeper into the subject and started gathering information.
The majority of the vibrations are typically from an out-of-true crank. True the crank up <.002" run out . Or <.001" if you can.
Smoothness is improved by lessening reciprocating mass. The lighter the piston, the better.
You don't need to do any adjustments to the flywheel balance.
Our forged 87mm 9:1 piston weighs ~472g including pin. If you can get the piston weight down around there, Or less if it is a 84mm, it will be fine. We don't change the flywheel balance.
Our Fireball builds run smooth as silk., and that's what we do to them.