If we're talking about simply dropping the AMG engine into the CLK 320, it'll probably bolt in without too much fuss. However, you'll need the ME-SFI control unit, and probably the transmission and traction control units as well. Obviously, you'll need the engine harness, and maybe the body-side harness. Do you have the AMG transmission? If not, the 55 V-8 wiil probably tear the original transmission up eventually. Same goes for the differential. Oh yeah, I almost forgot. You'll need the Electronic Ignition Switch (EIS) from the same car as the ME-SFI control unit came from. They're "married" together as part of the Drive Authorization Sysytem. Of course you can always buy a new EIS if the original one isn't available, but you'll need to order it using the VIN number of the donor vehicle. The original coil springs will have to go, and probably the radiator too. I can't think of anything else off the top of my head, but if I do I'll be sure to edit this post.

Oh yeah, don't forget AMG badge for the deck lid. After all, you do want this to be authentic, right?