ORIGINAL: donkey doctor
Hello; The way I understand it, the final port for the exiting (used) oil is in the bottom of the chamber containing the valve rockers. That oil won't go to the bottom of that chamber if the engine is inverted or side mounted, at least it won't until the chamber fills up entirely. I hadn't considered that the rocker chamber filled with used oil. That oil would eventually leak into the intake tract, and perhaps find it's way out of the engine through the carb, or even into the combustion chamber, if the engine stopped with the intake valve open.
You don't need to worry about it. The pressure inside the crankcase that's caused by the pumping action of the piston going up and down will drive oil throughout the engine. The front bearing will get lubed, the cam bearings, the valve lifters, the pushrods and rocker arms, and finally, the valve stems. Even oil will find its way up, inside the piston to lubricate the wrist pin/rod junction.
You may rest assured that the only problems involved with running any O.S. engine inverted are those people have had since glow engines were invented...if you don't have it broken-in and tuned correctly, you'll have difficulty with idle and acceleration because the glow plug will become the lowest point in the fuel system and tend to "load up" at low-to-mid-throttle settings.