Gasoline is the last thing you want leaking, especially on an 'in-frame' mounted engine. After the gas drips onto the hot engine, as you are cruising down the road at 30+ mph, picture the flames shooting up between your thighs, and swirling around your seat just before the explosion. Not a pretty picture. I suggest you get that leak fixed permanently, even if you have to buy a new gas tank, fuel valve, and gas-line.
As for your rear sprocket (that's what you call that rear 'wheel with the teeth'), normally the smaller it is you will have less acceleration but higher top speed...the larger it is, you will have more acceleration but lower top speed. I don't know exactly what setup you have, but if you are getting 40 mph and good acceleration with what you have now, I would leave it alone. That sounds like it should be about right. Please direct any further questions to the 2-Stroke Forum, or General Discussion Forum...that is really where they belong. Good Luck to you, and get that gas tank fixed before you become a statistic, Okay?
Patrick