There are many ways to upgrade your dirt bike from the factory settings to improve rider performance and machine performance.
1. I've always been a fan of a having a flashy bike. My favorite thing to do was to completely give my dirt bike a make over. New plastic, seat cover and graphics are usually the first thing to go. Of course this will do nothing to the performance of your machine but it will make it your own.
Next I check the obvious parts or the parts you can see. Tires, wheels,the pipe, chains and sprockets, slash guards and handle bars. Tires and handle bars are at the top of my list. I find that brand new rubber will really help increase the performance of your machine, helping you get deeper in the corners. Digging deep in the dirt gives you maximum hook up there by increasing speed and braking power.
I have always bought used dirt bikes so a top end rebuild is always a good idea because you never really know how whooped your machine is when you get it in this case. Sometimes you won’t need to replace the entire piston, sometimes you can get away with just replacing a ring (or two). An easy way to first impression to see what kind of shape your top end is in is to push down on the kick starter with your hand, if it is hard to do than you have good compression and your top end is in pretty good shape. If it is easy to do then you will want to look into replacing your top end.
Here is an idea of what I mean. This is my 1995 Honda CR125R
This is the before picture.
This is the after picture.