This is a subcategory for spoilers. This would affect the car similarly to the way changing rims does (changing part count, but not changing performance). This would override the part count/looks of the current spoiler, while not overriding the handling changes (and by extension, the race class changes). The cost would be 0.75x the normal cost, plus 50 scrap for anything but Equipped Spoiler (the 50 scrap prevents buying OEM [a.k.a. proprietary spoilers equipped by default] or No Spoiler for free).
That is a mock-up of what the feature would look like. How I did it? Hover over a high-tier spoiler and screenshot. (Edits: fixing the formatting, lol)