Horizontal shaft is the easiest way to go for a go cart. When I worked for a Hatteras yacht dealer, we were going to build a 1/5 scale version of a Hatteras 48' sport fish. Using twin 8hp Briggs engines centrifigul clutched to single speed f/n/r lawn tractor transmisions, scale water cooled exhaust, electric start, the 9 yards. Of course like most good ideas, it never made it of the drawing board.