piston rings

Good deal. I rated that as a long shot but an easy thing to try. Valve seals and guides typically don't get so leaky as to foul plugs, so your valve guides may be pretty worn. Keep and eye on it, and perhaps put a head rebuild in your long term plans.

Next step I would do is also easy: Take a can of Sea Foam or Rislone and slowly dribble half of it down the carb while revving the engine to keep it running. This will clean off intake valves and maybe free sticky rings. (It will smoke like heck and is quite entertaining too.) Put the other half in the crankcase and drive it to also help sticky rings, unless you have already added marvel Mystery oil or something to the oil. I think that a couple of oil and filter changes at 500 mile intervals are in order for your engine just to help clean things up a bit.

How many miles are on your engine?