When your tractor was converted to 12V, whoever did the conversion did one of three things:
Used the original 12V coil and added an in-line resistor. (Good)
Switched the coil with one that has an internal resistor. (OK)
Used the original coil and didn't bother to add a resistor. (Bad)
You need to see if there is an external resistor on your tractor. You can look for it, but you might not find it. You can test for a resistor with a voltmeter:
1. Turn the ignition switch on.
2. Read the voltage from ground to the coil terminal that is connected to the distributor. If this voltage is not zero, then the points are open and you'll have to bump the starter until the points close and the voltage drops to zero.
3. Once the points are closed, read the voltage from ground to the coil terminal that is NOT connected to the distributor. If this is 12 to 13 volts, then there is no external resistor. If the voltage is around 6 to 8 volts, then you have an external resistor.
The coil you need is a standard automotive coil available at most auto parts stores. Just bring in your old coil and they should be able to match it up. Not all stores carry internal resistor coils if that's what you need, but most will. Alternately you can add a resistor.