Finding switched 12 volt source

Mattax said
YR. Was it a CD box like MSD? In that case it wouldn't have cared about the ballast resistor. You would have just connected the small gage red wire to the ignition and the box's main feed would have been connected to the battery positive.

Yeah, this is a Mallory box but the wiring is basically the same.

This is why I ran a magneto in my race car. Besides a battery fired ignition can’t match the Mag, they are much simpler and easier to wire.