I couldn't hurt to add some "Marvel Mystery Oil" to the cylinder that has low compression, wait a few days, crank the engine over, add some more Marvel oil and then in a few more days crank it over a few times and then re-check the compression. I've heard it can unfreeze stuck rings on occasion.

 

Low compression could be valves or rings, the heads will need to come off regardless for the fix because you need to see the underside of the valves. But you can pick up a Borescope for cheap and go in through the spark plug hole and likely tell if it is obviously valves, cracked piston, or rings.
Oh, I'd do an oil analysis for metal just to make sure whatever failed hasn't contaminated the oil and you don't have bigger problems.

 

You do have to ask why only one cylinder might have seized piston rings with the others OK. Personally I think there is a valving problem. So before you lift the head, take off the valve covers and check all the tappet clearances. A large clearance on the 20 lbs cylinder indicates a valve isn't returning either due to sticking, or it's bent having hit the piston when the engine was revved. This might have been the reason the car was taken off the road all those years ago.

 

 

Yep the head will probably have to come off regardless.