piston rings

The cylinder-to-cylinder variations are decently good. The one cylinder that is fouling the plug could have worn rings but still have good compression because if the oil ring is bad then it will continuously oil the top and 2nd oil rings and make them seal well. So that part is not conclusive. Which cylinder is fouling?

I don't think adding oil and compression testing will make much difference but it is worth doing for completeness if you like; it usually is done if a cylinder is down to like 80 psi....

In my experience, a plug fouling is generally a bad oil ring. But changing the valse seals on that one cylinder is a quick job so I would try it. It might be that the seals are old and hard, and that particular one has completely fallen apart so that cylinder's intake valve has no seal at all anymore. So I would do that first; it is sooooo much easier.

If that does not work, then next I would suspect a possible sticking oil ring. I would take the plug out and move that cylinder down in the bore and pour in maybe half a pint of Marvel mystery oil and let it set overnight. Then turn the engine over (by hand preferably) with the plug still out to make sure the MM oil is not still in that cylinder; push it out as much as you can. Don't crank with the plug in case a lot of the MM oil is still in that cylinder; you don't want it to hydrolock and break things.

Those are the easy things to try first before pulling the cylinder head, pan, etc., and trying the ring change. You could also try a plug de-fouler/non-fouler to see how that works; I can't predict how good/bad that will work out.