It does depend of how old the engine is as to how hard. When you say magneto that refers to the flywheel. Now unless it has a crack or missing to many fins and not allowing enough air to be pushed around. Then I'd say its very unlikely that that is stopping the engine from sparking. If (where the magnets are) it is rusty then just clean up with some light grade sand paper or emory paper. I would say that something else could be wrong with it.
