First, what you call a Regulator, if it only has two terminals (BAT and GEN/ARM) IS NOT A VOLTAGE REGULATOR, its the older simpler two wire Cutout Relay. Many of them used a 3 brush generator versus the newer systems that used a 2 brush genny and a full fledged 3 or 4 terminal Voltage Regulator WHAT YOU REALLY HAVE DETERMINES THE ANSWER !!!!!!!!!!!
Q: Can the generator be converted to 12 volt
A: Yes, a competent old fashioned mom n pop (if any exist near you??) can convert a 6 volt genny to 12 volts, or else just buy a rebuilt 12 volt n be done with it, that may be easier n even cheaper. I've used 6 volt gennys to charge at 12 volts n did nothing (to the genny) but just changed to a 12 volt Voltage Regulator n they worked. Im NOT saying thats engineering correct or proper ONLY that it worked, the fields could overcurrent n get too hot !!!!!!
NOTE 1 you may well have a 3 brush genny as they were used on many of the older 2 wire (BAT & GEN/ARM terminals) Cutout Relay tractors!!!!!! Those are NOT the same as 3 or 4 terminal full fledged Voltage Regulators mind you. Soooooooo if you convert (or buy one) to a 12 volt 3 brush genny you can buy a 12 volt 2 wire (BAT & GEN/ARM) Cutout Relay.
NOTE 2 If you convert from a 6 volt 2 wire Cutout Relay system and 3 brush genny to a 12 volt 3 brush genny and 12 volt 2 wire cutout relay, THE LIGHT SWITCHES FIELD CURRENT CONTROL RESISTOR might have a different ohms value and if so, you need to also change out the Low/High charge Light Switch as it controls the Low or High charge on the genny.
HERES HOW I WOULD CONVERT::::::
I would toss any 3 brush genny,,,,,,get a 12 volt 2 brush more modern genny,,,,,,,,,use a full fledged 12 volt Voltage Regulator,,,,,,,,wire it correctly (post back if needed) do awy with any wire from the gennys FLD post up to the light switch if it has that older 3 brush system...........
Change lights to 12 volt,,,,,,,,,be sure n Polarize the genny,,,,,,,if you happen to be changing polarity, any ammeter leads need reversed.
Gotta run I will check back later for any questions