I have modified the fz50 moped. I have found that the bigger that you bore out the carburetor, the faster it will go. but you can only bore it out so much. I believe that if one were to adapt a 15mm carburetor it would be the perfect size. Another thing that you can do is replace the reed valve with a Boyesen reed valve from a Yamaha YSR50. The reed valve acts like a rev limiter at high speeds and this modification will help top end. Be sure to use Locktite. Also, you must modify the whole engine and transmission. I believe it is better to lighten the clutches than to change the springs. The clutches have mass and it takes horsepower to accelerate that mass. Not only that, the pivot points on the primary clutch is susceptible to wear so the less weight the better. when you modify the clutches make sure that that they are balanced. total weight and end for end, use a good scale. You will be spending a lot of time working on the transmission, so it would be a good idea to change all, if you can, of the Phillip head screws with allen heads. Pumping out an engine makes it age fast. I suggest using synthetic two stroke oil and changing the tranny oil every six months.