Miro has sent in a description and diagram for using 12 volts to start a 6 volt engine, without converting everything to 12 volt. Should be very useful. Thanks Miro.
Andrew, I finally put together the notes for the 12V start on 6V engines. Thought you'd be interested. I've sent it to a number of the guys. It's actually quite simple, straightforward , and much like an old marine engine.
It is attached in PDF format (click the Miro link below). You will need the free Adobe Acrobat Reader on you computer to view it.