I agree with Tiggs, to a certain degree. Insurance is based on "utmost good faith", which means you are obliged to tell the insurance company what you know about the modifications to your car.
The visual elements have to be declared, such as after-market exhaust, wheels, bodykit, even things like air filters which are obviously not standard.
When it comes to the internal workings of the engine such as cranks, rods, pistons, injectors, cams and the like, how are we supposed to know about these things if we have bought the car second-hand, with no real clue as to what has been done.
Do assessors go to the trouble of stripping an engine and researching what should be there and what shouldn't?
I have been reasonably honest with my insurance company, but I don't really know whats inside my engine, and I'm not going to take it apart just to find out.