There is no need to guess if your spark is too weak. Pull a sparkplug and ground the base with a clip and wire to the tractor frame. Shade the plug and turn the crank or spin the starter. If you get a visible spark, that is likely enough to at least fire the tractor up for starters. The old FM mags only put out 8,000 volts or so at their best, but it is enough with the low compression engines. It is important that the spark plugs be clean, non-resistor type for this reason. Also, the plug wires should be pure copper core, not the automotive carbon suppression type. If you don't have spark at the plug, get Old's manual or download it from the link below and follow their troubleshooting procedure. You can basically start at the outer cap and work your way in, looking for spark. If you have it at the cap, but not the sparkplugs, you have plug or wire problems. If you don't have it at the cap, but the internal copper rod gives a jolt, you have rotor or cap problems. If no spark at the copper rod, then you need to get into the condenser & points, or maybe even the coil. Those items you could replace. You will want to read up on timing the mags internal parts. If that wouldn't do it, you need someone who works on the mags regularly, such as Steve Barbato at email@example.com. The coil should not be riveted in, but held in by set screws that are removable. However, I have seen them chewed right up by previous removal attempts, and then you are in for a lot of work with drills and EZ-outs, or similar. Try a little heat and lots of penetrating oil.
But, unless you really want learn how a mag works, start at the sparkplugs and work your way in. If you have spark at the plugs, look elsewhere for the starting problems, like water or dirt in the fuel line. Doesn't take too much spark to at least get a kick from the engine.