Sorry, I can be of no help to you. Perhaps the lack of response may indicate that nothing can be fitted, or that that most things can be fitted. Perhaps a question about a specific part might help.
I think the clk was a 209 not a 208. You'd have to put down more information about both cars and what kind of things you want to retrofit. Anything is possible with time, money and inclination.
It must be a 209 and I was looking at retrofitting things like front seats, rears if it's possible, headlights look similar aswell just wasn't sure if anyone had fitted anything from a clk to w203 thanks for your help
The front seats 'might' fit, the rears wont because of the difference in body shape. The lights may look similar but again unless you can check part numbers and see that they are interchangeable, you are opening a can of worms.