Okay I have an idea, get a single throw switch to replace current kill switch (if it is momentary), have switch set so that when it turns on it sends power to the exciter wire. Have the switch also open a relay that is connected to the kill switch wire. That way you wont have to have two switches.
I sat down and sketched out how this would be possible without having a relay energized when it is shut down.
Though it IS possible to do this... in the end there are only 2 "simple" options:
1) Just add another switch and wire to energize and be done with it already
2) Sell the good working magneto on Ebay and convert to battery ignition
You really can't "tie in" to the magneto P-lead since IT IS NOT JUST A GROUND wire! It is actually part of the primary ignition circuit.