seems like you have an engine temperature problem, most causes are badly adjusted carburettor causing the engine to run hot (too lean mixture on load, your mechanic would mess around with the mixture screw but not know that the mixture screw only affects the mixture till 15% throttle opening), other causes are badly adjusted valve clearances (use feeler gauges) and do it only when dead cold.
the lower rocker arms also wear out on the pin they ride on, if those are shot then you will never get the engine to shut up, it will be really loud. Your engine is supposed to make a "machine sound" when warmed up with a little clackle of the tappets - It cannot be dead silent. I would also suggest that you remove the clutch cover and renew the two O rings of the oil pump as they must have hardened with overheating. If they fail you will lose oil pressure and you will have destructive engine wear.