Farmer, you ask "..... The gen that was used to replace the old 3 brush was a 2 brush. Is the 2 unit regulator not compatable with the 2 brush gen.........?"
Is what you call a "2 unit regulator", a Voltage Regulator (3 or 4 terminals) orrrrrrrrr a two terminal (BAT and GEN) Cutout Relay?????????? Is your Light Switch a 4 position LHDB (indicates a Cutout Relay) or just an Off Dim Bright (indicates a Voltage regulator)????
If its a full fledged 3 or 4 terminal Voltage Regulator, it may well not work properly going from a 3 to a 2 brush genny.
If its a two terminal Cutout Relay it can still work with EITHER a 2 or 3 brush generator
Once you figure that out and if its wired correct my Troubleshooting Procedure linked below may help determine if the fault is the Regulator or the Genny.
See if it will charge if you take a jumper and dead ground the Gennys FLD post??? If so that tells you the genny itself is okay so the problem is an incorrect Voltage Regulator or a wiring problem.