[rotorwrench]

As rings wear, the gap opens up for the changing circumference. The oil control rings are generally scraper types on these old engines. The modern ones with the expander are better but not infallible. Now days there are different rings available so I can't really comment on a type unless I can tell what type it is.

I don't reuse piston rings. I'd remove them all and check the dimensions of the cylinder walls. If they are still fairly close to normal specs then I'd hone it and install new rings. You can do the one cylinder if you like but it might be better to do them all unless they were recently done with low mileage & good condition overall.

On the valves, a lot depends on what type guides you have. If it has modern guides & valves then they can be removed and checked for stem wear and wear in the guides. A person can find a sticky one by doing that. If you have the old OEM mushroom stem types and two piece guides then removing them is a PITA. A person can release the springs and work them up & down to feel of the guides & stems for either stiff or wobbly operation but that's about it.

That compression was going somewhere. With intact rings, I'd sort of think there was a sticky valve or carbon crud hung on the seat and keeping it from closing all the way. Worn rings usually just give low compression but rarely no compression. Now a broken ring is different. That can destroy a cylinder wall pretty quickly. A leaky cylinder head gasket will cause overheating and extreme loss of coolant do to pressure in the system. It's easy to tell if the head gasket is blown.