Originally Posted by Venice Motor Bikes
Get rid of the rubber engine mounts! (they allow the engine to vibrate harder & move around in the frame)
Exactly what Venice said. Mounting the engine with pliable rubber mounts will cause the problem you're having. These engines are designed to be rigidly mounted. Get rid of the rubber and if that aluminum piece is in any way flexable get rid of it too and use steel. The more rigid you can mount the engine the less problems you'll have with failing fasteners.