Take a look at:
http://www.buggymasters.com/forum/showthread.php?t=1602 just to give you an idea- same principle for a standard chain drive.Most street bike engines won't allow the chain to go downward enough to get on the axle sprocket in a short space- so you either have to provide a jackshaft that moves the chain over to go down,chain roller/tensioner to redirect the chain downward(not what I'd do) or length in front of the axle sprocket by lengthening the swingarm. Looking at pics of that kart/buggy the rack or tube above the engine looks like it'll be in conflict with a taller engine too.You can save yourself some fab. time by finding an automatic clutch- I recall a few with some decent power, just a thought!