Good morning Farmer. Good question here's my response.
"Will I have to replace the 2 unit reg with a 3 unit type?"
1) When you state 2 unit regulator I "assume" its a full fledged "Voltage Regulator" with 2 coils a) (A) Cutout Relay and (B) a Combination Voltage/Current control relay. A simple "Cutout Relay sometimes used on a tractor with manual Low/High charge control system and sometimes older 3 brush genny systems, has but ONE coil IE the cutout relay. THEREFORE I assumed with 2 units (coils) you have a VR NOT just a simple Cutout Relay.
2) They also made 3 coil Voltage Regulators having a Cutout Relay,,,,,,,,A Voltage Control Relay,,,,,,,,, A Current Control
Relay.
Sorry absent any numbers I have no idea what you now have, maybe someone else has a method to do so??? Maybe some ID inside under the cover ???
THAT BEING SAID, my answer to your question is YES I would replace what you have with a PROPERLY MATCHED Voltage Regulator for the Generator you have,,,,,,,,,For the right voltage,,,,,,,,,For the right Ground (+ or -)
I have no comment on your choice of VR's or Gennys or Alternators etc. as that's YOUR choice and NOT YOUR question.
John T