If the engine is broken in, and you set the idle mixture correctly and don't expect too low of an idle, yes, it's doable. It's no different than running any other engine inverted, which is done all the time. You might find it idles better if you use an idle-bar plug, or a hotter plug, but you might not. It is worth playing with plugs a little though.

As for starting, I've seen guys do both, start them upright, or flip it over to start. Your choice. If you have a good cradle, starting it upside down so that the engine is upright and the plug lighter is easier to get to is probably easier and safer though.