Outdoor Power Equipment and Small Engines - vibration in Tecumseh 6.75 hp.

I got a freebie self prop. Craftsman lawn mower from my neighbor. He had put piston rings in the motor. It starts and runs ok. but there is a vibration in it that keeps loosening screws and bolts in the tin shroud. I've taken the motor apart and the timing marks seem to line up. Would it be possible that the piston was put in 180 turned from the way it came apart and this would cause vibration? There are numbers on one side of connecting rod and an arrow on top of the piston. Aside from the piston, anything else that would cause vibration?
Model #143.006700 eng. familyytpxs 1951BA
6.75 hp. Eager 1 Thanks

Could be caused by a bent blade or a bent crankshaft.