Had the same issue on a trip to Tonga
some years ago...reg got wet and didn't work.
I connected 12v lamps in parallel with a jumper wire so I could have between o and x lamps in the circuit and connected it between 12v and the field and watched the voltage carefully. As the volts started to climb I would take a lamp out of circuit to reduce the field current
I think if you used a potentiometer you'd need a big wire wound one as the field current
can be as high as 8 -10 Amps @ 12v that =100 - 120 Watts...!!!