Depends on how far the original owner wanted to go.
Via BMW dealers, you can get AC spoilers, wheels, interior trim, exhausts etc.
However, you can go the whole hog, and get AC (Germany) themselves to do an engine, gearbox upgrade (such as the case with Alpuina)
I think the car is then classed as an S2 or 3 dependent on the final spec applied to the car. These are, as you'd expect pretty quick motors.
try
www.ac-schnitzer.de
AG