[kz1000st]

So many things going on here. 50mm is a pretty good jump in size. Are you sure the piston isn't hitting the cylinder head. I'm not an authority on this but I recall someone needing to put a few gaskets at the bottom of the cylinder for clearance. Also are you sure the valve timing is correct? If the valve timing is out of sync your piston could be tagging the valves. The first test would be to put a socket on the center of the fan and rotating the crank back and forth to see if the engine is spinning to a point and hanging up or if it's locked.