You can run a 4-stroke inverted.

I have problems with 2-strokes--but I've run 4-strokes and seen others run them inverted too.

Just use the OS F plug and tune the low end right. They work great inverted.

Some common problems:
The carb/intake tube tends to get flooded when your fueling--so use a pair of Hemostats or some other clamp to pinch the fuel line going to the carb when your fueling.

Not leaning the low end enough. This is a lot of the problem guys experience with inverted engines.

Having your tank too high above the CL of the carb. If it's close---on a 4-stroke---your gonna be fine. But if the tank CL is more than 3/4" above the CL of the carb--you'll suffer many hours of grief.[&o]

Just use an F plug and try and get the CL of the tank close to the CL of the spraybar. It'll work great.

2-strokes inverted-----don't even get me started[:-]