Those engines never had a circuit breaker from factory. Circuit diagram below....
That said, if I were to add one, I'd mount it on the lifting eye plate at the back of the engine, above the starter motor (as they did with later engines) and wire it into the red wire that runs from the starter motor to the instrument harness plug. Be careful, there are 2 reds wires coming off that starter motor post, one is the one you want, the other runs to the starter slave solenoid... Here....
Here's how it's mounted on the later inline engines. (ignore the letters on the picture, there are for something else. This was the best picture I could find in the manuals)
