Captain
I did this about 60 years ago, so my recollection is vague,but I believe I left it in to run cause the magnets were shot and it didn't produce any spark without the aux battery. With the old style magnetos it makes sense that the current provided by the battery and interruped by the points is the same ? as current generated by the magnets passing the coil.
I don't have any hardware to look at but I envision something like a booster battery for starting only. You would probably have to wire it thru a diode to protect the electronics.
If you have one of the modern style solid state, CDI, etc systems laying around look and see if it has some common lead from the coil that a battery could be connected to.
Of course I may be all wrong about this.