The problem I have here is how easy it is to avoid the tax. CBP shouldn't have any jurisdiction over car modifications and repairs; that responsibility falls to different agencies.
The phrase you’re looking for is ‘criminal conspiracy’
If import tax on a cpu is x, and motherboard is y, should you have to pay computer import tax z if you build and sell PCs? After all, what imported cpus and motherboards aren't destined to be assembled computers?
It is a more normal way around these sorts of tariffs to import a vehicle as a kit of parts (CKD / "Complete knocked down"), and perform some level of final assembly in the end country. The sort of rules you talk about apply then, you pay the car parts import rate on the kit, not the (presumably higher) rate for a complete car[1].
[1] Or otherwise avoid whatever other protectionist measures mean you can't just import a fully built car.
