Have you disconnected the kill wire form the magneto, to see if it is shorted somewhere. The switch on a magneto grounds the wire to kill the engine.
There were a couple of different mags available. If you have an IH H4 magneto there is a cover over the coil on the very top that has a wire running to the center of the distributor cap. Remove that wire from the distributor cap (leave connected to the top cap) and hold it close to the metal case of the magneto (no more than 3/8 inch gap) and crank the engine to see if you have a spark. If not, you have an internal mag problem, points, condenser, coil, etc. If you do have a spark there, the problem is in rotor, cap, wires, etc. If you have a Wico, etc. magneto rather than an IH, the wire form the coil to cap is internal.
As a safety note, there is not a safety gap inside an IH magneto, so when you are trying to fire it, make sure the spark has someplace to go, such as having the wire close, using a spark plug held against the case, etc. Most auto parts stores sell a spark tester which looks very much like a spark plug with a large alligator clip on the side. You can even adjust the gap size to check the spark strength.
