The tractor was originally POSITIVE ground so I wonder if your new sold state cutout (diode) is installed in the right direction???? Some are labeled for Neg or Pos Ground, its simply reversing the leads, one to Battery/Ammeter, the other to the Gennys ARM post.
Isnt your M 6 volt?? you mentioned 12 at the shop??
If you polarize the genny to match your battery (Pos or Neg grnd?? Original was Pos),,,,,,,,Insure diode cutout is wired right (if not swap leads),,,,,,,,Insure wiring is right,,,,,,,,,Dead ground the field,,,,,,,but she dont charge I suspect the genny. See my troubleshooting and its gennyMotor Test.
If the diode cutout is right, it will pass current in the direction out of genny to battery/ammeter but NOT out of battery into genny. An ohmmeter will show near zero ohms in right direction/polarity but near infinity open if reversed.