Look under the piston. You may find a manufacturers name. If so you may be able to get a matching piston. If you use a piston with more or less meat. You are going to put your motor seriously out of balance. If I understand correctly. The small hole in the top of the piston designates (front). The hole goes towards the front of the motor. I would just use a Felpro 400 head gasket. With dished pistons and stock 76 cc heads a .020 head gasket isn't going to do a thing for you.