And that job is so much a bugger, be sure you are sure it isn't something else. Like a bad spark plug, or no compression.
For me the ultimate test is to stick a phillips screwdriver in the spark wire and hold the bare shank about a 1/4" away from the cylinder wrapper while cranking with the gas turned off. If you get no spark there from the screwdriver to the wrapper you can be sure it's a bad coil, but we always replace both and you should too. Be sure both coils are oriented the same way; with both of the spark plug wires exiting toward the rear (#1) cylinder. The coil with the longer wire goes on the front so the wire can make a u-turn before exiting the shroud. Also new magnetos have a rivet that interferes, which requires some quick filing so that it will sit flat. There's a kit that has both magnetos and a shutdown wire. And I'd recommend replacing the shut down wire as well. And your model is no fun changing magnetos compared to later models. Especially if you have mechanical governor. And you may also need the intake manifold gaskets replaced if you don't manage to separate the intake from the engine without the gaskets tearing. I spray the edge of the gaskets from the top with WD-40 before I begin. Petroleum products make paper much stronger than when it's dry. (Naptha, gasoline, oil, WD-40, etc.) And if the gaskets are stuck, then I try to take them off with only one light rap from my lineman's pliers. But occasionally one will tear and needs replacing. In which case, both surfaces need to be free of old gasket material. If you have a mechanical governor then care must be taken not to damage the linkage or lose parts of the linkage. On these older models I even use a universal joint in addition to the two foot long socket extension. Access is extremely limited, given that you don't really want to completely disassemble it, but just want to get it apart enough to sneak the magneto in there, and therefore dropping tools, razor blades, washers, and whatnots into the shroud is not really an option. I did leave a business card in one, I didn't go back for it.
State your problem, not your diagnosis.