How to Build a Rally Race Car

1). Upgrade the tires and wheels. Even in a stock class, there are many options for upgrading the wheels and tires. You can use any wheel as long as it maintains the stock diameter and width. Replace the tires with something better suited for the surface. Anything you can use on the road can be used in the stock class.
2). Replace the shocks and dampers with aftermarket units. This will give you better grip over the loose surfaces of a rally. It will also allow your car to better ride over the bumps and avoid damage to the chassis. You can also add a front sway bar to better control the front of the car if you need to, although this will tend to induce understeer which most road cars have anyway.
3). Add more power to the car. To stay in the stock class, you can change the air filter to a model that allows better air flow. You can also replace the exhaust pipe behind the catalytic converter, so long as the exhaust exits behind the driver.
4). Remove the spare tire, jack and tools to reduce the weight of the car. This will help both the handling and the speed.
5). Disable the traction control and ABS systems. This will give you more direct control over the car as you drive it. You can also adjust the alignment so long as no modifications are made to the factory range.