I've never weighted a flywheel just to balance it. Usually they drill out metal to do that. I've had them machined to lighten them, had them surfaced, and had them balanced, both alone and as part of the reciprocating/rotating parts of the engine you have balanced in blueprinting the motor. I seem to remember once that they had to use a slug of "heavy metal" pressed into a drilled bore to balance one that I had cut down. There are externally balanced engines that have counterweighted flywheels and harmonic balancers because crankcase size constraints won't let you put enough weight on the crankshaft counterweights themselves. 400c.i. SBC's are like that, where the 350 and smaller SBC is internally balanced.