I wouldn't use a fail safe as a good chunk don't work right with 2.4ghz radios. Also some newer radios have them built into the RX.
Throttle return spring is probably the only fool proof thing that will stop a nitro if it loses power.
As for remote termination of an engine... only thing I could think of is you would have to fit a 3rd servo on a controller with a 3rd channel to press something (like a piece of plastic) into the flywheel's rim to cause it to stop moving. You will need a fairly beefy servo though to do it, and concoct some rig. Or maybe some kinda electric valve hooked to the 3rd channel in between the exhaust manifold, and the pipe.
But of the shelf no nothing exists to my knowledge as it would have to be tailored to every single RC due to different setups