Well the engines do work inverted OK. unless you have a rare engine with the oil sump in it, or a float bowl style carburetor, they all work inverted OK.

But some caveats though;
   With most 4 stroke engines, the carb is positioned such that when inverted any fuel from the fuel tank that dribbles into it, will flow down into the intake manifold and into the engine if the intake valve is open. But when right side up, the fuel simply drips down and out of the carb.  A two cycle engine is the opposite, in that when inverted the fuel drips out and away, but when right side up, the fuel could flow into the engine's crankcase.

Then as mentioned, when inverted old oil residue or fuel etc can migrate down to the glow plug when the engine is inverted and the plane is in its horizontal attitude at rest.

But if you always turn your prop by hand first before you try starting the engine, these problems tend to go away as you can detect a hydraulic lockup before you try to start the engine.

Fuel tank location is always critical whether right side up, sideways or inverted. do try to get a good fuel tank position in relation to the carb. A Inverted engine position tends to be more critical of the fuel tank location and causing a siphon effect to start.