The question of "which spark plug to use on an old moped is quite common among your inquiries.
Indeed, the use of modern spark plugs very often seems to be the cause of breakdowns on our engines: misfires, sputtering, quickly fouled spark plugs, etc., etc...
In fact, the ignition system of our beloved machines is very rudimentary, and modern spark plugs are designed more for powerful, regular electronic ignitions and often for fuel injection systems... in short, the opposite of our old engines.