I bought an identical shell for mine on ebay - around £12-ish. Easy to swap the battery out, but the circuit board took a little persuasion with some needle tipped pliers. Took me a while, but my key is as new. The shell had a Merc badge on it, wasn't marked as original Merc, but was marked Huf, who make OEM keys etc, so for me it was good enough for me if its fake or not.
mercedes | eBay