Hi there, yes a car "Generator" can be repolarized, this means to swap the polarity of the output. In these "old" generators there was a two pole setup. the Pole shoes have copper wire wound around them, these have a variable current applied via a mechanical voltage regulator setup. But the short answer is to
1/ Make sure that there is NO POSITAVE earth equipment attatched to the circuit.
2/ make sure the battery is earthed to the NEGative (-)
3/ "flash" over the generator, just hold the positave lead and touch it to the
(F)connector on the generator, do it 3 or 4 times just to make sure.
(F) is for "field coils" it should go like a zittt sound
4/ Start the engine and get the generator turning, if you have an "Amp" gauge
there, it should show a gentle charge
The same thing can be done via the Current and Voltage regulator, but leave that to a Auto Electrician as welding to the current breaker cutout coil could take place.
30 years ago I was asked by an old boss to install a Jap radio into a english MG, the radio went Zitt and the smoke cameout and it needed to be repaired, so be warned