If you fly at an IMAA sanctioned event you are REQUIRED to have both a manual cutoff switch on the outside of the aircraft PLUS
a transmitter controlled method to kill the enging. It is covered in the IMAA Safety Code Rule 5 - to wit:

SECTION 5.0: EMERGENCY ENGINE SHUT OFF (Kill Switch)
5. Magneto spark ignition engines must have a coil-grounding switch on the aircraft to stop the engine. This will also prevent accidental starting of the engine.
This switch shall be readily available to both pilot and spotter/helper. This switch is to be operated manually and without the use of the Radio System.
5.2 Engines with battery powered ignition systems must have a switch to turn off the power from the battery pack to disable the engine from firing.
This will also prevent accidental starting of the engine. This switch shall be readily available to both pilot and spotter/helper.
This switch shall be operated manually and without the use of the Radio System.
5.3 There must also be a means to stop the engine from the transmitter. The most common method is to close the carburetor throat completely using
throttle trim, however other methods are acceptable. This requirement applies to all glow/gas ignition engines regardless of size

Some inspectors will want to see that your manual switch works and some will also want to see the transmitter kill function also. I have found the inspections
vary from event to event.

I use either an opti_kill, an IBEC or a magneto kill from the transmitter on all my gas engines.

Dan