I have never had a 2F apart so I can't speak from direct experience on this one, but I have had a bunch of old school Honda motorcycle engines apart. Many have long cam chains and jumping a tooth isn't unheard of. When they do, the valves contact each other or a valve and a piston contact, causing some disintegration/breakage. Likely cause of your plug damage. I would get the head off and take a look to see what's in there jamming in the valves.
Oleg has a rebuilt 2F for sale