Such a definition is moot if you could sue for a court order then go to a dealership with a sheriff's deputy in tow and take just the part you wanted out of a brand-new car, and the dealer could in turn recover all of its costs for that now-unsellable vehicle from the manufacturer.
There would be a great incentive to warehouse and sell "spare parts". And they would have to be every individual discrete part, and every subassembly that can be handled as a unit. If you just sold engines, you might end up with an unusable nearly-engine because someone came along and picked just the parts they needed off of it. If you just sold engine parts, you might end up with an unusable nearly-car because someone picked a whole, assembled engine out of it.
So every type of screw, nut, bolt, and washer. Every type of transmission assembly and engine assembly. Every passenger seat. Every floor mat. Every light bulb and every plastic housing. Every wiring harness. Every radio and CD player.
This is itself a great incentive to use COTS parts, reduce number of unique parts, and adhere to standards. Maybe you use a #6 bolt somewhere instead of trimming weight and cost a tiny bit by using a #5 there. Maybe you standardize the dimensions and mount points of your engine compartments, so that 20 years from now, replacement engines can be diverted from the supply chain for newer cars rather than pulled from a warehouse stockpile that has been taking space and gathering dust that whole time. Or maybe you set up your spare parts facility to do on-demand manufacturing or assembly.
It is a big can of worms. But I'd argue that not opening it now would be allowing it to grow even bigger and wormier as time goes on.