In reality, once cars get modded they rarely, if ever, get returned to standard, it's just too much work and too much cost, especially when resprays are needed to cover things like arch mods (see Karl's example).
And the the spare parts rarely stay with the car. More likely the owner gets sick of tripping over them in the garage, needs a few quid one day, and sells them. Think back, Esprit S1s were chopped about like this once, hence all the "Wolfrace wheels wanted" adverts you see now people want them back to standard.
Your car obviously, but I see an unmodified S4 or GT3 as on the endangered species list.