RE: Honda GX31
In a 4 stroke engine the crankcase is filled with oil that is circulated by an oil pump... The oil pump is located at the bottom of the crankcase when the cylinder is pointing upright (on this particular model)... Thus if you mount this engine inverted you will starve the oil pump and your engine WILL fail...
On the other hand a 2 stroke does not have oil in the crankcase... the crankcase acts as part of the intake system, which is why you have an oil/fuel mixture so that the moving parts inside the crankcase recieve the lubrication they need....
The other thing is that yes, the oil in the crankcase will seep past the rings and fill your head full of oil if left sitting for even just an hour or two...
Unfortunately 4 strokes all have this limitation... So I personaly would not attempt to mount this motor inverted...
Now... if you were to find an engine that could be mounted on its side.. and that was a suitable set up for you, then that would be the way to go...
Otherwise I would find myself a good 2 stroke as they don't really care which way you mount them....