This is going to hurt any way you do it. The most practical thing to do is find a good 86 or 88 sedan, and then restore it as they are almost all in sorry shape. A new transmission is likely available from Mercedes but the price is going to be ridiculous - more than the cost of a whole sedan or coupe. I would look into a European 5 speed coupe and then federalize it. This is not to say this is a practical idea but you would at least have a factory drive train on a rare nice car, except maybe for the rust.