Magneto Advice please

[Bob Olive]

As it starts/sparks from cold, it might be that the winding insulation is breaking down when hot - I've had several mags do this after a good start and then twenty to thirty minutes running when it starts misfiring, and then fails to restart. They did however start later when cold, with the same result after running for a similar time.
Try leaving it running on the bench using a lathe or drill with a jury rig full set of plugs for about half an hour or more. That might prove something.The safest thing to do of course is to get a (fully equipped) mag specialist to vet it thoroughly, though even they can be fooled sometimes...