Metal is probably from the key and transmission wear. Clean it out good and flush the bottom end. Change the tranny oil after it's back together after each break in cycle. Should clean all the junk out. I use Maxima 4 stroke 5/30 in my trans, flushing it use pennzoil 5/30 in the silver bottle. As for piston i'm guessing it's a 67.75mm bore piston not stock. Get a 68mm Wiseco piston and then have it bored and honed to that piston. Cometic gaskets. Get pics of the piston please.