Sounds like it's been a bit lean, detonated and blew the head gasket and f**ked the side of the piston. The HG failure may also have damaged the head, although again, it would be interesting to know why the old head was deemed scrap.
Piston likely to have melted a little and/or trapped rings, or broken ring lands. Often neither of which leaves an easily visible sign on the crown..

Although at the same time, there is usually some sort of indication on the plug.