The spinner / propnut, two or three bladed prop, drive washer, shaft with counterweight, drive washer and reciprocating piston and rod need to be taken into account as well.

You have two rotating ends of different masses joined and separated by several inches of crankshaft of non uniform density.

I'd guess thats another reason why it's very difficult to make the counterweight effective, besides the difficulty with a single cylinder engine.

I vaguely remember the idea of making the prop-driver balance-able being tried with the 1960's OS 15 Max 3 glow.

OS didn't further pursue the idea.

Ray