Look at the battery cable terminal where it attaches to the starter. There should be another, smaller wire under the same nut that attaches the battery cable to the starter. That's the power feed to everything else in the electrical system, lights, ignition system, etc. Just isolate that wire, and attach the 6V lantern battery + terminal to it, and the - terminal to a good ground like the engine block. Now, use a 12V battery and a pair of jumper cables to run the starter- - - -problem solved. The starter will handle the 12 volts with no problems, and the ignition and other functions will still be on 6V.
Jerry
