Martin, the piston cannot hit the top of the head, it's probably hitting the piece of bearing cage or roller that got left in the engine after the initial repair. You should be able to feel this as you turn the engine over with out the spark plug installed, as the piston comes to TDC you will feel slight resistance then with a little more pressure it will go over TDC. If the crankshaft bearings feel tight, there is no end play in the crankshaft, and there is nothing leaking from the main bearings, you should not need to remove the prop hub or crankshaft. I would carefuly inspect the bearing on the big end of the rod by using some magnification and turning the bearing around the crank throw with a small screw driver through the oil slot in the rod. If anything looks marginal continue with the disassembly.