Fairbanks-Morse Magneto help

"Generally" the problem with any mag type system is the points, condenser, or coil. In the case of "some" mags there are various rotating devices--such as for the rotor--that add complications

(In the one off our old Farmall Regular, the points actually rotated when it ran, very tough to troubleshoot that old girl}
Points MUST be in good shape and have very low resistance
Condenser MUST be good and have LOW leakage and you can not test that easily nor with a multimeter. A new condenser does not mean a 'good' condenser
So far as the coil, they can have "partially" shorted windings and still pass the ohmeter test and if they have ingressed moisture, you might be able to bake them at low temp for a few days then if they work re-seal them with appropriate sealer but this is a iffy situation.

If the cap/ rotor are dirty/ moist/ carbon tracked, they will attenuate spark.

Also the magnets may be weak. Some of them can be "recharged" but good luck finding someone to do that

Mag parts of that nature can get expensive quick Only thing I can suggest is look around in your area "for some old guy" who knows mags and really knows how to make them tick.