Firstly these bikes have many restrictive components which mean that the pit bike can not perform at its best all the time. The most restrictive parts on these bikes are the carburettor and the manifold. A carb can be picked up for as little as £40-£50 which is a bargain for the potential power it could give you, and when you have purchased the carb it would be good to invest in a performance air filter, which allows more air to flow into the engine which equals more horsepower!
Next it would be an idea to change the exhaust to a performance exhaust, this combined with the carb and air filter will mean great power gains. Exhausts can be bought for around £50 off some websites but then again some of these exhausts retail at hundreds of pounds so be sure to shop around before making your purchase.
These two modifications will add low to mid range torque with a good benefit to the top end of the bike and a small increase in overall horsepower.
In a lot of pit bikes there is usually a restrictor in the CDI unit, these are often limited to around 8,500 rpm, if a performance CDI unit is purchased then this will be increased to 10,000 rpm meaning a smoother and more powerful acceleration curve and a higher peak meaning better top end as well.
Adjustments can be made to the gearing of the bike as well, in order for you to adjust this depending on the style of track you will be riding on. If you want to make adjustments to the gearing of the bike the best thing to do will be to change the sprockets. Standard sprockets are usually 14 tooth on the front, with a 37-40 tooth sprockets on the back.
Changing the front sprocket with a larger one will result in better top end, and a smaller rear sprocket will have the same effect, however acceleration will be lost when top end is gained and vice versa.