Pierre,
Unfortunately the original rubber trunk mat for the 230ies with upright spare wheel is no longer available from factory.
Currently, there is no vendor around with reproductions.
I have the same problem like you, mine is rotten.
Basically, you have three choices:
1. Look for a good used one (best solution), but hard to find.
2. Use a custom made carpet trunk mat (e.g. from squareweave carpet)
3. Use a new style rubber mat and cut out the area for the spare wheel well. However, you will still have to fill in the place on the right side of the trunk (= "small mat"); it will be visible.
Best,
Achim
(Magdeburg, Germany)