Any gas engine will basically need 3 things to fire. Fuel, spark, and compression. I would first make sure you are getting spark by removing one of your plugs and turning the engine over with the plug touching the side of the cylinder head or somewhere around it that has ground. Make sure you cover the spark plug hole by draping a rag over or something to prevent too many fumes in case it does spark. 
Verify this with both cylinders, if you don't have spark, test 12v lines going to the coil, etc. If you do have spark, make sure you are getting fuel. Last resort could be compression check to make sure something inside the engine did not break somehow.
If it ran fine before flipping it, my guess is it's not getting spark or you got water in the crankcase. Maybe someone around might have a little better idea off hand. Jet a couple basic pointers. Good luck