I think any basic waterproof push-button, wired to ground when pushed, would work. The button on the works "hack" 650SS I rode, when working at N-V, saved my bacon when the throttles jammed wide open in rush-hour traffic. I'd have ended up among the passengers on the rear platform of an old-style double-decker bus otherwise.
That particular bike was magneto ignition. Whether grounding a coil/breaker system would burn something out, i'm not sure.