LiveSteam and anyone else who is interested.
The voltage regulator would not be directly connected to the output it would be connected in series with and control the field coil only, that is how the original type regulators are connected, the full output of 20 - 40 Amps at 12 Volts would then be available .
I have guessed the current requirements of the field coil, the first thing I would do if I had a suitable dynamo is to check the resistance of the coil to enable me to select a suitable electronic regulator.
I would also set the dynamo up on a Lathe or Drilling machine to get an idea about the speed needed for 12 Volts.
A quick Google suggests a speed of 1500 - 2000 RPM is needed for maximum output.
Below is a diagram of the original setup.
