But have you considered the car insurance premium factor when modifying your car? If you own and live in a large private estate, then modify all you like. As long as you drive within your estate, you don't even need a car insurance. But supposing, like most people, you love to tinker with your car but drive them on public roads anyways? Then you need to worry about insurance premiums, because then insurance premiums go up.
The reasoning goes like this: you can modify your car in two ways - you can either modify its engine or whatever else that makes it go at a greater speed, or you can modify its decorative and comfort providing parts, like seats, polish, whatever. If you modify your car to make it go at a higher speed, it is reasonable to assume that it will be made to go at a higher speed; ergo, it is more prone to accidents and your premium goes up. In truth, any change you make to your car engine will certainly increase your premium.
You can modify the other parts of your car as much as you like. You can polish, draw pictures, add bumper stickers, whatever you like. Oh, but wait, there was a recent study by a psychology professor from some Midwestern university which claimed cars with bumper stickers are more prone to accidents!