i agree with that guy ^^^
save up and get a bigger bike, my DT is running a pod filter, 60 bore kit, performance expansion chamber and exhaust, pretty much everything i could find for it to make it go faster and it still only tops out at 50, cruises at 40, ive poured atleast £500 onto that bike looking for a bit more comfort, style, speed e.t.c but there isnt much you can do with it, pass your test and get onto a real bike
the cheapest and most effective way to get more speed is to put a larger sprocket on the front, you will loose a bit of acceleration however and you need to make sure your engine has enough power (usually 2 teeth larger on the front is adequate)