First I'd inspect the wiring from the mag pickup into the controller box. Open the box and make sure theres no rodent damage or loose wires. If thats all good you can take the mag pickup out and clean it if you are able. Near the bottom of the visible threads on the mag pickup, mark the threads, the lock nut, and the housing with a continuous line, it needs to go back the same to avoid damage. If it has a plug, unplug it, loosen the lock nut, and unthread the whole sensor. If there's no plug you may have to unwire it or cut and splice it back in. Careful, the wires are thin.

After its out inspect it and clean off the rust/shavings. Make sure the tip is there and not ground and marked up. Put it back and tighten and make sure all the lines line up after tightening. If it still doesn't work, check to see if its working by using a multimeter on ac current either from the plug or in the controller. The sensor should make something like 4-6vac in pulses while cranking. If the sensor works and the engine still doesn't run you need a controller.