Does the engine have a magneto or electronic ignition? Either way, you will need an external physical kill switch. On the Magneto equipped engine, the switch grounds the ignition and kills the spark. On an electronic ignition it disconnects the battery from the ignition box. Having a way to kill it with the radio is necessary too. On some engines you can reliably shut it off by completely closing the throttle. Having a physical servo-actuated switch to ground the magneto or an optical kill switch for electronic ignition is a good idea though.