"Tecumsehs were known for sheared flywheel keys that were just enough off to make it not run."
That's why I asked the question. New flywheel probably wasn't the problem in itself; Alnico V magnets last a lifetime. I don't remember the brand of engine when I experienced the partial sheared key mystery.
What amazed me was how sensitive those engines are to ignition timing. I guess that's the reason ignition points were still being used when small engines went from a separate coil to a flywheel magnetic pickup. Must have been lower voltage outputs as compared to today's machines. Higher voltage would mean a steeper leading edge with more control on the positioning thus eliminating the need for ignition points. Never pursued the reasoning and stages of the transformation.