David,
As Doug says, the balance factor is less important on flat twin crankshafts than on singles or parallel twins. Provided it hasn't been modified, the crankshaft itself should still be balanced as it was when it left the factory. What may upset the balance is the possible replacement of either the conrods or the pistons - they should be fitted as matched pairs to get a perfectly smooth running engine. Checking the weight of the pistons is easy, but the conrods have to be supported in a horizontal position so that the weights of both the small ends and bigends can be checked and matched.
-Eddie.
