Not sure a "button on the wheel" is what I'd go for. Let's say you crash, etc and are still moving, and the wheel is making like "round and round" due to the ensuing action I would want a great big switch somewhere you can reach easy, say near the shifter, away from damage that might occur
So far as 'what to break" depends if you are EFI or carb, it would be handy to kill the fuel pump along with ignition. Also if you have a trunk mount battery (or even if you don't) you can buy bulk lengths of bike/ motorcycle/ morse cable and rig a "pull" release for a battery switch. Bear in mind killing the battery ONLY will not kill the engine if the alternator is still connected.
