I'm guessing that if it is o-ringed for that age bike it is a modified head, they came with a gasket from stock but had a real odd set up with little to no squish band.
What does the piston look like? it is very common for the exhaust side of the piston to be damaged from water entering the chamber if it was running. Check the rings they maybe stuck in there grooves, and they go out of shape quick.
I o-ring a lot of heads, the o-rings tend not to fail in fact I've never had one fail. It will be a reaction to something else that will course the o-ring seal to fail.
So you may have to find the course before you can fix the problem.