Well, before I saw your two posts I picked up a Universal coil at TSC that was designed for 12V tractors with no external resistance. The tractor fired right up and seemed to run fine though I couldn't test it under any load because I'm working on the seat right now and it would be a bit uncomfortable with the seat off.
I figured that I should start with this coil because I figured the other one designed for an external coil would provide a very hot spark and wear out the points quickly if my tractor didn't have an external resistor (and I certainly couldn't find one).
At least that's my primitive understanding of the ignition system. I guess if I find that it is hard to start in cold weather or with a low battery then I might have to reconsider whether I have the right coil. But with all the cranking I did before I figured out the old coil was bad I think the battery was pretty low when I tested the new coil.