The engine doesn't have to be re-balanced when swapping frames. The engine is balanced eternally (Crank/Rod/pistons/balance shaft). You will need to make sure you follow the very specific engine bolt tightening sequence and also make sure you properly tighten the frame to engine spacers on the inside of the two back frame bolts. Vibration can occur if this sequence is not followed properly. It is also advised to make sure you take all of the weight off the frame when re-installing the engine and going thru the proper tightening/torquing process.