i didn't miss the fact he wants to stay magneto ignition. there are plenty of ways to skin this cat without an extra switch.. etc.
so.. is the machine hand crank? or push start?
it has no button to engage an electric starter solenoid.. even a manual one?
lotsa options I can think of on a diesel or mag ignition unit.
i'm not a fan of 1 wire units. generally cost more and may take igher rpms to excite.. and some have high quiescent leakage when not running.