Now that you have the dirt bike you've always wanted it's time to make it faster and more powerful. You need to get more fuel and air into the motor while getting the exhaust gases out faster. Replacing factory parts with aftermarket parts and having your motor machined will get you the speed you need.
Make a list of parts you would like to purchase from reputable aftermarket suppliers. The three most common upgrades are a bigger bore and stroke kit, port and polished cylinder and heads, and a better reed block and reeds. Replacing these parts will make your bike much faster. Do some research to find out what work you can afford and which parts you can replace. Beefing up your bike isn´t cheap.
Start with getting a bigger bore and stroke kit installed. These kits improve horsepower in the mid-range and overall power band, allowing for increased power that's easier for you to control. The bore and stroke kit will include an after-market cast cylinder, piston and head, and aftermarket crankcase. The installation will increase your motor size, giving it more displacement and more horsepower.
Port and polish your cylinder and cylinder heads while the motor is out. A machinist will drill out your motor so it's smooth and clean, meaning air will move through the engine faster. The overall effect will be an engine that runs stronger, smoother and crisper.
Replace the reed block. A standard reed block controls the fuel to air mixture ratio. A custom reed block will enable the engine to get more air with less effort. This will improve low-end and throttle response, meaning more horsepower with greater acceleration.
Take your bike out on a test run to make sure everything is running smoothly. Once you're sure that everything has been installed correctly and the bike is running like it should, test your speed.
Hire an expert if you can't do the mechanical work yourself. Installing dirt bike parts requires skill and if it's done incorrectly it could result in an accident.
A fast bike is fun but it is also dangerous. Remember to always wear protective gear and a helmet.