the fixing of the points cam holder uses a special “washer” with two grooves and a hex-head.
The points cam holder has two “noses” that match with the grooves.
It may be, that the “noses” are broken off. Not nice, but not a catastrophy.
It is recommended to turn the crankshaft using the larger sized washer head (17mm wrench) and not to use the smaller sized (12 or 14mm?) screw head itself.
Rotation direction of the crankshaft is backwards. Counter-clockwise, looking from the ignition side. As the screw is a normal “right hand” screw, there is a chance of loosen the screw instead of turning the crankshaft.
One of the “gimmicks” in that engine.
Good luck !
PS: I had a damaged piston once and can’t remove the piston rings. There is an undescribable “melting mixture” present, that “glued” piston and rings together.