The only real problem is when the engine (having a glow plug in the middle) is straight upside down and you load it up with after-run oil or flood it through the muffler (hydraulic lock). When it is canted to one side, the plug usually doesn't get squenched in oil if there is a little too much inside.

Be aware of the hydraulic lock possiblity. If there is too much resistance in pulling the engine through compression to prime it, you may have to pick up the airplane and turn it so the muffler is facing the ground, then pull it through gently to clear all the liquid out of it before attaching the ignitor or trying to use a starter. Many connecting rods and wrist pins have been broken due to hydraulic lock and not checking for it, before trying to start up.

I have several inverted engines, both two and four strokes and have very little trouble starting them sitting on the ground. Just don't over prime. Watch the fuel line going to the carb and just get the finger over the carb (two stroke) slightly damp as you pull the engine gently through compression. Then hook up the igniter and start as usual.

good luck.