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Originally the 1956 Johnson 30 HP. was 6 volt. But over 60 years some things could have been changed on your motor. The 6 volt starter motor will operate on 12 volts with no problem as long as the engine is in good operating condition and starts normally. But if you have to crank the engine abnormally long to get it started over time you may have an issue. I would look to see if your starting solenoid and the electric choke solenoid on carburetor are 6 or 12 volt. Originally all parts were 6 volt. If you want to operate engine on 12 volt and the electric choke solenoid and start solenoid are 6 volt I would replace them with 12 volt parts. you could use the parts from a 1957 engine as this is when they changed to 12 volt. I have a 1956 30 HP. and started it it for years on 12 volt I just would choke it manually on engine and not use the electric choke and my solenoid was an original 6 volt and there is a screw cap on the bottom of it that could be unscrewed to reveal a manual solenoid button that I used to crank engine. Never had a problem. I now use a 6 volt battery for the sake of originality but it is awkward if you want to use 12 volt devises such as fish finders or radio’s Regards Bill,