Does that have a voltage regulator mounted right on the underside of engine cover that you flip up to access the motor? If so, it probably is a old mechanical type with points and that probably is causing the voltage to jump all over the place. That is a Delco system like they used in GM cars back then and you should be able get parts at a auto parts store.
P.S. Is the battery running down? Are you measuring the voltage at the battery?
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