Little Progress
I finally found time to measure the adapter. Got a 3D dummy to avoid mistakes in the original.
Not everything fits yet, but it looks good.
I borrowed the wheels from the neighbor. He is not allowed to drive right now. Bent his Boxster a bit. (alcohol).
The tire is a bit too wide (255-40), but it is possible to try it on.
225-40 is enough for me.
For the front I have to go a very expensive way because a specialist accepted the order and received the parts, but neither executed the order nor sent the parts back and did not buy them at all. (to replace the replacement value).