In the UK? No, you insure the car as a whole. And if you dont declare your mods, and the insurer finds out (like if you want to claim for anything, or have a claim made against you in the event of an accident) they will likely void the policy from inception. Meaning you would get hit with driving without insurance charges as well as the cost of any third party repair. And your own car wont get anything from the insurer. Not worth not declaring.
I say this from 11 years working in a claims investigation dept in car insurance.
Insurance companies are not charities and they are perfectly entitled to know exactly what they are insuring so they can evaluate the risk and they almost certainly have research relating to the likelihood of a person making a claim based on among other things
age of both car and driver the drivers age and history,where they live,make and model mods and type etc.
IMO you don't seem to have thought things out fully as your 500 quid car can crash and will crash into vehicles with much higher value like say a RR,Bentley,Porsche MB,BMW or into a shop fence house etc and if you are found at fault your insurer have to pay for the repair the other parties vehicle or property as well as a hire car while the repairs are carried out