To support what has been said so far, here is an image of the 2 different types of magnetos, I have just refurbished and tested a bunch.
[Linked Image]

Yes, the silver ignition modules do fail,.
I set up a test engine, gap and bolt the magneto down, earth out the module and the lug with a clamp lead, make sure the kill wires clear and then flick the flywheel magnets past the magnetos armature whilst earthing out the spark plug and look for a nice arc across the plugs gap.

When servicing I do this prior to mounting the engine.
I agree with above, it is rare for magneto coils to fail, the ignition modules fail more often.
Check all you can before condemning the magneto because in a lot of cases it is fine.


Happy is he who penetrates the mystery of things.