RE: Counter-rotating props on a single engine?
You would need to end up with a hollow drive shaft with an internal shaft turning the opposite direction to get contra rotating props. You're going to need gears etc capable of running at the max rpm you expect your engine to develop and you would need to keep it fairly light. Sounds like you've got your work cut out for you!
The Tupolev TU-20/TU-95 "Bear" uses contra-rotating props... not quite sure how it was set up. Unfortunately none of my books are very specific about the "plumbing".I remember seeing something about a gear box for an OS engine, but I think it was a gear reduction drive... not sure how you would adapt it for use with contra props...
I'd suggest making friends with a really good machinist! Good look with your project!