I ran my Old john Deere tractor with the three brush generator and cutout relay system just fine for many years. It will charge and maintain the battery just fine. The third brush is adjustable and is used to set the generator current output. The problems with these setups is that there is no current relay in the cutout.
You can certainly put a voltage regulator in the system, and then crank the third bush up for max output. You will however notice that the generator will not charge the battery at low engine RPM. This is typical and nothing to worry about.
The other option is to get a simple 6v "intelligent" battery tender. They are designed to stay connected all the time, and will not overcharge the battery, but will allow it to cycle a little bit and keep it "topped up". Really great if you don't use the vehicle much. Batteries Plus sells them for a reasonable price. About the size of a laptop power supply.
Kurt
