I'm not sure about the Ute, but a USA stovebolt engine got a different flywheel ring gear as well as a different starter when the conversion to 12 volts was made. 6V flywheels have 139 teeth, and 12V flywheels have 168 teeth. Both starter drives have 9 teeth, but the angular pitch of the teeth, and possibly the diameter of the gear is different. A 12V starter running against a 6V flywheel makes a lot of noise, and both gears wear quickly due to an angular mismatch on the teeth.

I'd recommend going back to your 6V starter. It will work OK on 12 volts, as long as you don't keep the starter engaged for a long time. The choice of generator or alternator for battery charging won't make a difference about which starter to use.
Jerry

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