You wont know if the cylinder plating is shot until the piston is out and the cylinder is cleaned up. Used stock top ends are cheap though, so that's not a real issue. The condition of the crank will tell you whether you need to split the motor or not, but lets face it, if the piston is that corroded, the crank is probably rusty too. I'd tear the whole motor down, clean and inspect the parts and put the usable ones in a box, then order replacements for the rest.