Of course the other likely thing with old motors is it has "thrown a leg out of bed" or in other words, has snapped the con rod and the motor will just free spin when trying to start. Check to see if the bottom pulley is turning when you pull on the starter. If it is, it's new motor time.
Good catch Aussietrev. Now that you mention it that could be the source of the loud bang. Should have thought of that myself since a I rash of failures a couple years ago where the operators at a nursery here was breaking rods left and right from not keeping the oil level checked. Rod journals were overheating and seizing the the crankshaft journal. Usually destroys the motor as the broken end takes other things.
Usually you still fell the weight of the flywheel spinning though or could be just experience that teaches you the feel. Maybe I have been at it too long...
