outboard magnetics in flywheel?

The magnets are oriented so that the magneto coils will get peak energy at TDC to fire the plug. If the magneto is mounted in a different position then the magnets will be located somewhere else. Some engines have multiple locations where a magneto can be mounted.

If you put the wrong flywheel on, the plug will likely still fire, but the energy of the spark will be much lower and probably not enough to reliably ignite the fuel.

If you cannot move the magneto, you might be able to cut a new keyway into the wheel using a saw. Energy is transferred to/from the flywheel by frictional force against the hub; the keyway only serves to align the parts.