I've messed with old HD's that have set up for upteen years and it sounds like stuck rings to me. Spray alot of PB Blaster in the plug hole and fill the oil tank with motor flush,wait an hour. Then leave the plug out and kick it over for 5 or 10 min to circulate the oil through the engine. Drain it out and fill it up with fresh oil and filter. Kick for 5 min,install spark plug and fire it up with the tappet cover off to see if oil is flowing from the head. Let it run till warm and drain,replace oilfilter,refill oil tank.
It sems like it's a long process and costly but how much does it cost to rebuild an engine?
Used oil will turn into acidic sludge in the bottom end and tar in the top end and oil lines. Trust me,been there,done that.