You are clutching at straws; i will explain why... when a crankshaft is balanced it is done so with all ancillary components attached, flywheel, pulleys, gears, retaining bolts; the clutch plate is left out as its position is not fixed. The crank assembly is then dynamically balanced; flywheels and crankshafts are generally pre balanced separately first. I am assuming rotax balance the 912 crank with conrods attached; i could be way wrong. Adding a balance master to the rear without adding also to the front will serve no benefit.