The problem with mounting 2-strokes inverted is that if they run too slow or too rich, the glow plug will get a puddle of liquid fuel and oil in it and go out, causing the engine to quit.

Generally, you will need to keep the idle a bit higher, and your mixture settings will be more critical than side or upright mounting. A hotter running plug (maybe even a 4-stroke plug) can help some.

Also, if you manage to flood the engine, it becomes basically impossible to get the liquid fuel out of the engine without either removing the plug or flipping the plane over, since there are no ports or anything at the top of the cylinder. On the plus side, you won't have a problem with a syphon effect filling the engine as the carb will also be pointing down, allowing fuel to drip out of the engine inside of into it (assuming normal front carb design).

Gassers don't have these problems becaue the fuel-air mixture is much leaner than with glow, and the spark always happens, it doesn't rely on just the right amount of pre-existing heat and reaction between the fuel and plug element.