There are two sources of power for the exciter circuit on this machine. The main one takes output power from the generator then goes through the voltage regulator back to the rotating field to continue the cycle of generating. The regulator varies the power back to the rotating field to keep the output steady and correct. The other circuit provides power to the rotating field while you are starting the engine. This starts the loop. It is similar to flashing the fiels on old cars that had generators.
It power from the starting cycle portion seems to be working. This tells me the rotating field, the brushes and the main winding is all good. The problem is most likely the voltage regulator or associated wiring. There are people who repair and return these as they seem to give quite a bit of trouble. If you google the part number of the circuit board, you will probably find a few of these folks happy to repair your board. I have repaired a few myself as it is almost always the same components that fail. Dust,dirt and moisture seem to build up and cause the fault.
I hope this helps. Robert