I have to say something about salmonbug's comment about engine orientation. I understand what he is saying that an engine will run fine no matter how it is turned and he is absolutely correct.

However, it does make a difference which way the engine is turned because it changes the carburetor location in relation to the fuel tank. Ideally what you want is the centerline of the fuel tank to be slightly below the centerline of the needle. One of the easiest ways to accomplish this is to turn the engine until the relationship is right and then mount the engine in that location. Of course, while you're doing that you also have to ensure the prop shaft is on the thrust line.