Are you already drilling it out?
I had the same thing on my 190E. Key didn't even turn to align the holes for removal. Then I got a very helpful hint from this forum to vibrate it.
Insert the vallet key (steel on without plastic). Use something to generate vibrations on the key. I my case this helped and I didn't have to drill anything out.
I ordered a new ignition lock on vin number and got a shop tumbler during shipment of new one. I did only change tumbler itself.
The tumbler may sit thight there so do not give up on it on the first time swhen you insert removal wire tool. And it's very important that you got a diameter on the wire that is as large as possible. Otherwise it willl be harder to get locking plate pushed down. Also, if I recall right there should be only a locking plate in the upper hole.
Click here for my thread
BTW: Cosworth developed the 2,3 16 valve engine for Mercedes. This should be the same engine as any 16 valve US cars. And same bodykit.