Well Cal,
Here is the trick. If the generator has an adjustable voltage regulator, it should be able to charge an 8V battery if the generator is 6V, and you go into the regulator, with the engine running fast enough to produce electricity, you can adjust the voltage up to enough to keep the battery charged. Most 8V battery manufacturers recommend a charging rate of 9.2V. It might be hard to get that high, but you can get close, and it will work. If everything else is right, the 8V is the trick that will spin that engine. It works real good for me, all my gasoline tractors have magnetos, the extra cranking speed relates to a hotter spark, and easier starts. It can't be used if the generator is 6V 3 brush, that would have a cutout relay instead of a voltage regulator. If I wanted to, it is possible to remove the 3rd brush and holder from the generator, rewire it and replace the cutout relay with a voltage regulator, but for me that seems like alot of unnecessary trouble to be going thru. And it does change the original appearance. I would opt to go 12V on the 3 brush 6V setup. I still see lots of 6V systems out there, working just fine, but you have to keep everything just right, sometimes I find that hard to do, especially last minute trying to get to a show situations. The big drawback to 8V is finding a battery charger. 6 or 12V is easy, you might need to build or modify one to charge 9.2V. If I have a dead 8V battery, I generally crank by hand or pull start the tractor, and let it run until the ammeter reads zero, that might take a while, but I get to listen to that sweet engine running.
Take care!!!
Andy