you've got 2 options really...
1. Cheap (ish), dynojet the original ypvs carbs, by all accounts this works well, but at 70 to 80 quid, seems a lot for a spring and needle and jets, and fit a freeflow airfilter.
2. expensive up to very expensive, get a pair of carbs (amal,mikuni, dellorto) probably 34mm or 36mm
then have lots of fun setting them up, routing the cables (can be a very tight fit under the tank, and in some cases the tank might need a bit of 'persuation' ) but when it's all sorted will give you better performance (and higher fuel consumption) and more induction noise depending what air filter arrangement (if any) you use.