My suggestions: Get a new head+cylinder+piston+rings, or just throw your old engine in the metal recycling bin and get a new one.
Most likely you have worn out your top end due to poor maintenance and/or bad luck.
Eg.
* Dusted it via the air intake.
* Poor oil change maintenance and oil in there which is either cooked/thinned or full of soot and clutch particles and blocking oil galleries up.
* Over revving/loading the engine when cold with your thick 20w50 oil.
* No magnetic oil drain plug so fine metal particles have totally blocked oil galleries in the head.