Get yourself another piston. some guys just have a couple kicking around and can use an old piston, but you're not going to have that luxury. So a new second piston it needs to be. Wrap the end of the piston with sandpaper and make a big fat drill bit out of it. Spin the piston in the head until the head is the same shape as the piston. Get good sandpaper, SIA comes to mind, and cut narrow bands and stretch across the piston.