Crankshafts are deliberately made 'out of balance' so that when it is connected to the rod and piston the reciprocating balance is zero. Crankshafts, of their own, don't get unbalanced. If the front bush wore, then it is more likely down to something coming in with the air through the carb causing abrasive wear, a bent crankshaft from a previous crash, wrong oil, or not the right amount, use of an 'electric finger' damaging the thrust washer and causing bits of fine metal filings to get between the bush and crank or a badly balanced prop, although you discount the last possibility. There isn't really anything else...
Evan, WB #12.