Hey Cal,
In my experience with magnetos the most common failure is the coil. A bad condensor could cause the problem, but I would have the coil checked. A dead give away is, do you see any black gummy residue inside the mag? If so, the coil has gotten hot and the tar has leaked out of the coil, when that happens the windings have no insulation, and a short circuit occurs. There can be other things go bad in a coil that give no obvious indication, but that is the most common. I use a firm, Branson Enterprises in Love Park, Illinois to test my coils, they will want you to pay return shipping, and will have just about any mag or carb part you need, within reason.
Have a great day!!
Andy