Is it converted to 12 volts? I presume not. You’ll want to disconnect the gas gauge for sure if you’re using 12 volts.
You can wire it up temporarily just to get it running by bypassing the ignition switch. Make sure the hot lead from the battery goes to the starter button. From the starter button you want to send power to the coil through a ballast resistor. That should be all you need to get it running, but this method also bypasses the generator so you will not be recharging the battery. Also make sure your grounds are good. Don’t leave this connected without the engine running or you might burn out the points. Dont step on the starter for more than about 10 seconds at a time. Running on 12 volts makes it heat up and it needs time to cool between starting attemps.

I'm sure if I'm wrong about any of this someone will come along and say so shortly.

1948 3600 thriftmaster