ORIGINAL: ChuckW
Anything is possible if you want to bad enough. The propane cylinders those things use are pretty heavy though. Maybe some sort of smaller, aluminum cyclinder would work better.
Like Minnflyer pointed out there is a lot of work involved with making it work. I have a 30cc 4-stroke powerhead for a string trimmer that I was going to convert for fun but decided against it.
I also doubt that the performance would be as good as a comperable displacement purpose-built RC gasoline engine.
If you want a cheap gas engine, look at some of the Chinese stuff out there right now, not bad considering the price.
Of course it isn't always about practicality. Sometimes tinkering and having fun is nice.
From what I found they use a splash system of lubricating. My old '53 chevy pu was like that.
Inverted flight prolly wouldn't be a big problem nor would most aerobatics. Radial engines have about 2 or 3 upside down cylinders and they do just fine. You would have to be careful about startup if mounted inverted though. A radial has to be turned over by hand several times to pump out the oil that settled in the bottom cylinders or it would hydro lock and break a piston or rod.