To an extent...
You can modify the engine with Turbochargers and other upgrades. You can change exhaust systems and tyres and you can reduce the car's weight. You can even put new wheels on the car.
However, you cannot give the car Racing Car modifications ie. modify the appearance of the car to make it look like a racecar.
(Though I do like putting new wheels on my RUF and lowering the suspension!)
