ORIGINAL: pmw
Engines without float type carbs such as our model engines will run in any position. That's why we can fly inverted all day long. The issue is always tank to carb placement. If you can get the tank located correctly, an inverted engine will run fine, assuming you don't flood it during start. Otherwise you will need a pump for consitent runs.
The issue is also the head of a two stroke engine's deepest point (when inverted) is the glow plug's location. Becasue of this, this is a natural well for fuel to migrate too. On ocasions, this will cool the glow plug to the point of killing the engine. Because the head shape and glow plug location is not necessarilly the deepest point on an inverted four stroke, in most 4 strokes this is not as big an issue.