Mil-specs are very stringent. Should be a tough motor. I like the rope start too.
Mil-spec is nothing special. Some of the specs are very stringent, many aren't that hard to meet. For true rough service I would expect a flathead engine, not overhead valve, especially for an engine made in the 80's. In the 3rd world, flathead engines are the norm for small gas engines due to the ease of repair and rebuild.
We don't know what the original specs that this engine had to meet were. Obviously the engine would have had to meet power, fuel efficiency, weight, reliability, and cost factors, but we don't know in what order they were prioritized in.
The biggest thing I would be worried about is access to replacement parts. In the military, parts are never a problem
Just trying to advice caution, just because something is Mil-Spec doesn't mean it's automatically good.
Now onto the plus: After some research I find that the manufacturer, Teledyne also owns Continental Motors, which is a major manufactures of piston aircraft engines. Aircraft engines have used overhead valves since WW1, so the company that made these knows how to build OHV engines.
I find the compression ratio is listed at 6.0 to 1, which is something that could have been accomplished with a flathead engine, however they must have used OHV to improve the flow in the engine which would improve power and fuel efficiency with minimal impact on reliability at the RPM range this engine operates at. The low (for a 4 stroke) compression ratio was done to improve reliability by reducing the stresses on the moving parts and reduce the temperature of the cylinder head. It appears to use pushrod OHV which implies a gear driven camshaft, as used on many aircraft engines and NOT a chain driven OHC one, which is also good for reliability.
I also find that these originally sold to the USG for $893.00 each in 1987, so either someone made a killing on kickbacks or these engines were well built.
So, at first I was skeptical, but now, upon further research, that these engines might be something to look at. I still have concern about replacement parts, but at least the companies that built them are still in business.
Now just to figure out how the hell I could hook that up to a bike.