The speed of the car on the road in miles per hour is limited by the size of the engine and how much power is transferred from the tires to the road (i.e. torque).
If you have a limited budget, then here are some first steps:
1. Replace the factory air intake filter and plastic box with a K&W cone filter
2. Replace all the ignition wires and spark plugs with a high quality brand
3. Replace the tires with an expensive high speed rating brand
This will give you a small improvement but there other options in modifying the engine that will cost far more money than it is worth.