As long as the carb faces the right way these motors can run in any position imaginable.
A friend has an OOOOOOld 60" bar chainsaw that actually has a float bowl carb on it. The carb is part of the handle and if you want to turn the saw on it's side you push a button that releases the handle and lets you rotate it so the bowl is still facing down.
Why don't you mount the engine on the rear? I did with a friction drive motor. I just welded some extra arms off the swing arm for the mount. Can be seen at SB worksmam site. Kind of like Thats DAX rear mount