Okay, it is possible to convert an old Cutout Relay that used a Manual Low/High light switch charge control over to a full fledged Voltage Regulator but this is NOT going to be a perfect original genny to control system conversion BUT I'VE DONE IT AND IT CAN WORK. In the old original days the VR was matched to deliver the correct amperage to match the gennys Field windings specs/rating ya know. If the genny has a third adjustable brush when I made this conversion I usually adjusted the third brush to the max charge location.
WIRE THE NEW VR AS FOLLOWS
BAT up to Load (NOT battery/starter) side of
the ammeter. Thats the side that used to wire
to BAT on the Cutout Relay
ARM to Gennys ARM post
FLD to Gennys FLD Post. NOTE FLD on Genny used
to wire to light switch but you dont do that
now, Gennys FLD now wires to VR instead
LOAD feeds battery voltage to loads like lights and ignition (via their switches). In the old system those loads were fed instead off the load side of the ammeter (same place that wired to BAT on CR) but if you convert you feed them instead from L on the VR. NOTE it will still work if you dont do this and leave them fed from the ammeter.
The VR has to be suitable for the Tractors ground which was originally POS ground. Be sure the Genny is polarized correct before start up or you could fry the new VR.
PS if the VR accidentally overloads and overheats the gennys field windings dont blame me, but if its a 6 volt VR suitable for the tractors ground it will most likely be okay