Will it keep a battery charged? Of course it will—if you don’t take more juice out of the battery than the gen puts back in. Running a trolling motor would be a good example.

People will be quick to point out that the gen "only" puts out 10 Amps. I submit to you that many/most more modern motors only put out 5 or 6 Amps. But of course the other side of the argument is that the gen puts out nothing at idle.

In your search, the regulator must be for a DC generator, 12 Volt, 10 Amp, "A" field type. Harley did have such a thing, no car has only a 10 Amp generator.

I would submit throwing this mud in the mix…. for the most part, when would you need the genny part operating? If the motor is setup properly, it would require very little juice to start. Once started no juice would be required. I would guess to run lights, but as Frank stated, it only puts out when at high RPM, so running your lights at night would be off battery solely as in my neck of the woods idle speed after dark.