What do I do to convert my TO-30 to a single wire alternator?
If you convert the tractor to 12 volt you must wire it as 12 volt negative ground. That means the coil should be a 12 volt coil with an internal resistor. There are two different resistors used with coils. As I said, you need a 12 volt coil with an internal resistor. Coils come with internal resistors and external resistors, and this has nothing to do with the resistor used to drop the operating voltage of the coil. The positive terminal on the coil should be connected to the wire going to the ignition switch, and the negative terminal on the coil should be connected to the side of the distributor. You will also need either a diode, small light bulb, pressure switch or resistor to prevent electric current from back feeding through the alternator. See the wiring diagram on the FENA web site for particulars.
I would also suggest replacing the ammeter with one rated at 60-0-60, and using a minimum #10 wire from the alternator output terminal to the ammeter, and then to the starter switch. There is no difference between a 6 volt and a 12 volt ammeter, but you do need one that will handle the alternator output. The original 30-0-30 with a #12 wire is not heavy enough.