welcome to the forum. you should be able to get about 48-52kph,if your running a 26'' wheel with a 44t sprocket. a smaller sprocket will give you more top speed,but reduce acceleration. if you want it to last dont use a nos setup. a simple performance air filter and tuned pipe from(sick bike products) should give you little more power. i would not do much more than that to these 2smokes the more stuff you do to em, the shorter the life span. if your motor has a slant head(angled spark plug) those engines usually have more compression.
There is a fine line between a hobby and mental illness