Normally, I would not recommend this: he could power the starter (only) with a 12 volt source. A jumper cable would work: negative lead to the body-to-clutch-housing braided-ground-cable and the positive lead would have to be touched by hand to the positive post on the starter (disconnect the 6v positive lead from the starter). The ignition key would have to be turned on to provide power to the coil.
Be careful. If the starter does not turn easily, the engine might be too tight or the starter might be weak. I think that forcing 12v through the starter for long periods could damage it.
I trust that my suggestion above will be corrected and/or shot down by others with more expereience.
Tim
