50cc dirtbike improvement?

Ok so I have a 50cc dirt bike. Some chinese model. I have had it for about 5-6 years now. It tops out at about 15mph alittle less on flat smooth pavement. (I'm 130 lbs) I want to make it go uprange of 30mph. 30-35 would be plenty fast. I heard you could change the carb and the fuel jets on these little bikes for some more speed. Would it be difficult? I have some experiance with small engines(weedeaters, lawnmowers, mopeds). Would it cost alot? I'm 13 and don't have alot of spare cash with all my other hobbies(fishing, hunting, R/C). Also the stock muffler is heavy! and I mean 10+lbs. Is there a performance muffler out there that is cheap? Do I need a muffler at all? (it sounds pretty awesome idleing without one). Thanks for helping and links to ebay auctions or websites would be appreciated