Mick it sounds like your mower is in need of a really good service, it is very hard to diagnose the problem without looking at and listening to the machine but i will give you a few tips on where to start looking.
Firstly remove the spark plug and make sure it is the correct plug, next make a diagnoses of it,if its black and sooty with particles of carbon adhering to it it may indicate piston rings etc are worn and you may need an engine rebuild. If it is a brown/tan colour this normal.If it white/grey it indicates the fuel mixture is too lean which may be caused by an air leak somewhere, possibly a loose carby manifold, mounting bolts, gaskets etc.
The overheating problem may be caused through poor ventilation around the motor, are all the cooling fins on the cylinder and head clean and free of debris, grease etc.
Next check the oil, is it at the right level,
in these light engines without an oil pump the level being too high can be just as bad as being too low.Also is it the correct oil SAE 30 Monograde. Not car engine oil. The correct oil is available at any mower shop.
Does the mower a popping/banging noise when starting/stopping or changing the engine speed this may indicate burnt valves or a muffler full of holes, also check the muffler gasket and mounting bolts.
Next check all of the spindles/ bearings and belts of the cutting deck, a motor would be working very hard to drive a belt on a seized spindle bearing etc, you should be able to turn them all by hand.
I hope you find this information useful.
Regards Eddy.
