First thing Id try is see if the light switch itself has a good case ground to the tractor cuz if not field current cant flow through it or its resistor (if its okay???) to ground i.e. NO CHARGE
If the switch has a good ground it ought to at least charge in the High Charge setting cuz thats there even if its field resistor were bad. If its grounded okay but wont even allow field current to flow through it in its High Charge setting, then the switch may be badddddddd.
Those switches arent all that expensive and cheaper then changing over to a full fledged Voltage Regulator so test its ground and see if she will ever charge via the switch.
NOTE there MUST be a good wire and good connection from the Gennys Field post up to that switch also ya know,,,,,,insure its good n in place cuz if the switch isnt connected to it she cant work.
The genny will charge highest with its Field post dead grounded but only charge about half with a 2 to 4 ohm or so resistance in series from Field to ground (if youre gonna jury rig it)