This is all assuming you have a battery ignition and not a magneto.
The battery igntion, even on a 12v tractor, operates on 6v. So you will have one of two setups.
A) a resistor inline between your ignition switch and the coil. This reduces the input to the coil, which is wound internally to make your spark from a 6v input. In this case, a failure of the resistor will leave you with no input to the coil. Generally, in my experience with ballast resistors, it just fails, and stays failed. A bad coil often, but not always, will fail as it heats up, when a weak spot in the windings heats up and breaks continuity in the windings, breaking the circuit/field. In those cases, it will often close back up when it cools and run again until its heated back up. If you wire around the resistor, you'll be delivering a straight 12v to a 6v coil. It will take it, but it's hard in the long run on points and condensor. Okay for test purposes, but not general running.
B) In the absence of a resistor, you likely have the other type coil, which is made for a direct 12v input. It doesn't actually have a resistor built into it -- it is wound differently inside from the 6v variety, to produce the same spark.
If the tractor is showing the symptom of dying after ten minutes or so, but restarting and running fine after an hour or two to cool down, before dying again, I'm suspecting the coil. Someone else made the very good suggestion of checking for a spark from either the ignition wire from teh coil or one of the spark plugs ASAP when it shuts down. If no spark then, but an hour or two later, you can figure coil (especially if it is WICKED hot to the touch or seems to have been leaking oil or other stuff). If no spark later either, the condensor and coil could both be suspect.
As far as replacement coils, you have your choice. If you have a resistor, you can leave it in place and buy a 6v coil. You could also eliminate the resistor and buy a coil that takes a 12v input. If you have a 12v input type coil already, just replace it -- I don't see much point in adding a resistor and switching back to a 6v coil in that case.
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