And along with the SKREEM and Key problems, there is the PCM.
Searching for additional information WRT the SKREEM, the KEY and the PCM (ECU), I found an interesting video showing details on how to clone an ECU (PCM).
The video shows a cloning process for another Mercedes model, not a SLK320, but the technology is the same.
It can be done, as demonstrated by the Merc Tech. However, it requires skilled Micro Electronics surgery, specialized equipment and software.
The tech removes the CPU chips from the current and donor ECUs, places them in a chip reader to copy the original CPU data then write it to the donor CPU.
Then, he re-solders the CPU chips back into their ECUs and, Voila!, he has 2 identical ECUs.
So, it can be done, BUT... We need someone with Micro-Miniature Repair skills, the equipment to read/write the chips, and will to take the risk of bricking your PCM.

All in all, it's probably better to watch for wrecked and parted out Crossfires with matching and working Keys, SKREEMs and PCMs to salvage as spares.
Cheers,