Standard rear bumper for this model/age (with or without chrome):
Aero rear bumper with 'S' centre exit - Cooper exhaust would remain at the side (also shows Aero skirts):
As the Aero kit was never actually "standard" on the Cooper (or any first gen model for that matter - it was always an optional extra), I would declare it to your insurer. Stress that it is a dealer fit option though, and not an aftermarket kit.
You just need the skirts (and possibly the rear bumper) now.