Coming from an Insurance professional in CA
Other states besides CA have aftermarket parts coverage, although it does cost a lot like others said before.
Although Insurance companies split up the term Aftermarket parts.
Of course, we all know there are Aesthetic and Performance mods you can do.
Anything that tampers with the engine internals that is not OEM parts being repaired or replaced are Performance Mods.
Aesthetics are of course body kit parts etc.
Insurance companies see Performance mods as a reason to break traffic laws since it makes your car faster and more dangerous, seen as much more of risk that they exclude it from their policies. Since it is a intent to make the vehicle faster. If you are in a wreak they will place the blame on your for the intent of making the vehicle faster with performance mods making it unsafe and a high liability risk.
The grey area I don't know about are Aerodynamic mods. Like Diffusers and splitters and canards. They are aesthetic, but for performance. So if you have it and any insurance person asks you they are for aesthetic reasons lol
So for example, if you strap a Twin Turbo Kit to stock/performance internals and 3 months after blow your engine, your insurance company will deny the claim because of them being performance modifications.
Even if you were in an incident to where it was a collision and not a blown engine, the insurance would pay out aesthetic parts but not the TT kit or any install/labor.
I have not found one insurance company other than Lockton Motorsports (track insurance) which would cover Performance Parts. But that kind of Insurance for $100,000 costs close to $500 per month ($6k/yr) here in Cali.
IF YOU FIND A COMPANY THAT TELLS YOU THEY COVER PERFORMANCE AFTERMARKET PARTS LIKE THE TT AND SC KITS GET THAT SHIITT IN WRITING LOL!
Just my
