Converting to 12 Volts

The problem with using a resistor is that not all coils draw the same current, so a certain value converting resistor that works well on on type of coil may be too low in value for use with another coil, causing rapid point wear, and an overheated coil, or with another coil the value may be too high, causing weak spark.

If you insist on a converting resistor, a "real" auto parts store should be able to come up with one to try. When you get it running, check for 5 to 7 Volts on the resistor (switch) side of the coil.

Apparently you have tried a couple of resistors on your tractor?

The way a resistor works, it has to be powering a load (Current flowing through it) to show a voltage drop across the resistor.

You must have checked the voltage on both sides of the resistor with the breaker points open, so there was no current flowing in the ignition circuit. That is why you got the 12 Volts in and out.

If you still have one of the resistors, try it again, and start the engine, and use an analog voltmeter to check the voltage on the switch side of the coil for the 5-7 Volt range I posted before. If you are within that range, you SHOULD be OK.

The reason for using the analog voltmeter is that a lot of digital meters will be affected by the EMF produced by the ignition and charging system, and may go "nuts" and read incorrectly. An analog meter is less sensitive to the electrical "noise".