The Group N rules do not allow any performance modifications, only safety modifications such as rollcage, fire extinguisher, etc.
About the only performance bits you can fit to a Group N car are the exhaust and filter element and even then it's not supposed to make any more noise than standard.
If anything Group N cars are slower than showroom cars because they are heavier. But, engines for GpN cars are rather special beasts really.
You take one engine, look at the manufacturers blue-prints then machine the engine so that you are just inside all the manufacturers tolerances, thus getting the best performance. This is known as blue-printing an engine.
There can be a lot of work done in 'correcting' things like the cylinder heads (although this is not necessarily permitted by the rules!)