As the F1 car went on to become the SLK Black, the parts would be available through Mercedes Parts at any dealer.
However, I think the F1 car did not have any rear silencer boxes, but not sure if Mercedes would not have put something in for the Black albeit an excuse for a silencer to keep some of the noise down. The only other mod they would have done is a cross piece which is available off the shelf.
You might be better off getting an exhaust specialist to make up a system for you.