Or if you have a 2 stroke with electric start, you could start it backwards. Won't run very well due to ignition timing but if you balance it just enough to get it to run enough to move then you will have success haha. FYI, 2 stroke golf carts do this, except they generally have 0 degree timing I believe which allows them to run equally well in either direction.
EDIT: Just thought of a stupid but novel way to have reverse on a kart. It wouldnt be that effective and would add heaps of weight, but would allow very basic backing up. Basically It comprises of a wheel on a lever on each side of the kart, some sort of mechanism would link them, and a bottle jack or something would power the motion. The arms would push the wheels up against the normal drive wheels and for it to work would have to push the whole kart slightly into the air. This would mean that the usual drive wheels are now friction driving another pair of wheels. These new wheels would rotate in the opposite direction to the normal wheels, thus providing reverse......Personally though, I'd rather just get out and turn the thing around instead of stuffing around with that.
EDIT 2: Just realised what you actually asked. Theres a search button near the top of the page, left of the log out button lol.