Either way you stop the engine (or all 3) would work.  You either starve the engine of air, starve it of fuel assuming it's NOT ingesting lube oil,  or starve it of an ignition source (compression).  The recommendation is not to stop the engine with the compression release but I bet there's also a recommendation not to let it runaway either so better to have a dead-man's comp release (starve of compression) and a shutoff on the air intake (starve it of air).

That Murphy overspeed tach device looked pretty neat, a little more high-tech than the rest of the engine but if it kept you and the engine safe it's probably money well spent.  It has contact closures when the configured overspeed RPM is reached.  having a ball valve with a weight attached on the intake and a solenoid holding it open would be fairly simple.  The Normally Closed contact could hold the solenoid (and the ball valve weight) from shutting off the air.  As long as the RPM were within limits the engine runs.  Overspeed the solenoid either pulls out (active) or lets go (fail safe) the weight drops closing the ball valve and the engine shuts down or at least slows to some less dangerous speed.

I wonder if a low-tech method might work, something heavy mounted on an attached platform that would be shaken loose during an overspeed attached to a chain that could close the ball valve.  You'd need something that couldn't creep off the stand during normal operations but during an overspeed when vibrations and harmonics were at their worst would walk off and pull the chain.  That wouldn't need any external power at all.

Very interesting stuff, I love this forum.

Robert

I've seen people ask about turbocharging or supercharging but never seen it in pictures but might someone at a high elevation be able to hook up some sort of turbo device or supercharging device to run off the belt to give a little extra HP at the higher elevations?

I would think the increase in HP would outweigh the parasitic loss to the blower and might allow for a higher amount of usable energy being delivered to the ST head.  Of course fuel use would go up accordingly.

Perhaps a blower assembly using an automotive A/C clutch could be put on for the times when load outstrips supply.  flick the switch and engage the blower and let the engine think it's at sea level for the high load period.

Robert