Well I found the culprit....broken exhaust valve.
piston looks ok....appears not to have done any damage, probably because I was just cranking and not running.
Lucky for me the previous owner had a number of spare parts and gave me the old head....going to rob the rocker arm from that.
Going to rob a valve as well....looks like it's in good condition....no putting on the seat. Going to get some lapping compound and see how it goes.