Okay so i just bought a kart with a 5hp briggs. i want to know some cheap quick mods to the engine that will get me to about 60mph because 25mph isnt good enough. it is a live axle kart.
We've got a 5hp flathead class at a kart racing track here that most everyone gets up to 60mph top speeds. You'd have to do all kinds of expensive mods but it can be done. If your serious, you should spend some time researching at this kart racing site: http://karting.4cycle.com/forumdisplay.php?f=1
Have a look here for a guide on modifying your Briggs engine.
As for getting 60 mph out of a Briggs for $50? Well you can gear it really high which may get you there but you'l have crap acceleration and you can forget about hills or dirt or grass.... You'l be burning out clutches too.
If you want a properly fast go kart then get a motor bike engine.