You don't need to kit a bike just because you put a bigger carb and exhaust on than stock. Some of the exhausts and carbs are made to deal with the higher volume flow necessitated by a kit. For example, most of the time, you would never need a 20mm carb on a stock one speed bike. That's just more air and fuel than your engine can effectively use.
Also what was mentioned before is important, if you increase the size of your carburetor, you need to increase the size of your intake as well. It doesn't do you any good to put a 15mm carb on a 12mm intake. It also doesn't do you any good to put on a bigger carb and intake if the intake port (the hole that the intake bolts to) is too small. That's where porting comes in, but porting is tricky, figure out exactly how your engine works first, then start thinking about porting.
A bigger carb and intake, as well as a performance exhaust can go a LONG way as far as making your bike go faster. Eventually you might want to change your gear ratio for more torque or top speed, or mill the head down for more low end. But a pipe and a carb+intake are a great place to start.
Do you know if your bike is a 2 speed or a one speed?