So-yea, basically you can only put a few choice modifications on your car, ie map/exhaust/brakes. Nothing like larger turbos or any other massive engine modifications.
As per Wayne's post, it's not a matter of choice of different types of modification - this isn't Need For Speed. You must fit this camshaft, these tyres and these dampers and springs to make your car compliant with the regulations. Otherwise you don't race, because your car doesn't get through scrutineering. These items are offered to the racers at a discount, so the whole field is 'levelled'.
The MKII Golf GTI series runs under very similar regulations, as does the Locost and most other 'single model' championships.