Do you have the spark plugs out?
The headgasket won't catch the piston regardless since the piston edges do not protrude past the bore.
Did you align the balancer shaft with the crank? There are alignment marks on the sprockets/gears so that they are "in phase". I can't remember if the a lobe on the balancer shaft would contact the crankshaft if it's out of line. Maybe @
Racer x knows more.
So the crank hangs when it hits 2T every 360* of crank rotation?