Good spark at both cylinders would rule out sheared key/flipped cam. You mention the fuel is fresh being used on other engines, is it regular grade fuel or high octane? I know it is counter intuitive (at least it is to my simple mind), but these low compression engines do not like to run on high octane fuel. I had to prove it to myself several times on my fleetwin when I thought I was being smart using high octane av gas.
The engine won’t even fire when you squirt fuel in the plug holes? I’m sure you have explored the possibility that the problem is too much fuel/flooding. The only other longshot I could think of is the exhaust snoot is plugged up with mud, but this seems unlikely if the engine quit while underway. Do the plugs show any signs of water? Have you tried dry firing it for a second or two (not long enough to damage impeller)? I realize you don’t have a compression gage up there, and these things don’t have much compression anyway, but does it feel like it has compression? It surely couldn’t hurt to just try the head bolts to see it they will loosen up, nothing to lose. You could always use some gasket sealer on the old head gasket if it isn’t blown, I realize you don’t have an extra head gasket on hand. The job could be done right after vacation is over. You are SOL either way if the head gasket is blown, perhaps someone close by could send you a gasket up there.