What about aftermarket Radiators? Preferrably with aluminum sides, not the thermal plastic. I've ran into the exact problem you posted, and so far, I've seen no radiators with the aluminum inserts. I know the mounting holes (for the bottom posts) for Mercedes are the same on the 190D, and the 250D.
If there aren't any aftermarkets available, isn't it possible to buffer the radiator by using a strip of padded foam/door weather stripping where it attaches to the radiator support cross beam, and the bottom area?
Also, as a note, if you have one of these radiators, and chose not to replace it, here are some things to know.
1. The nozzle tends to break off and leave a "nub" on the radiator. In a pinch, you could re-attach the hose without leaks, but you better find a shop fast, and take it easy on the car.
2. Unless you can take the radiator apart and put it back together, you can't fix it. All known sealants, and glues (including silicone, and epoxy) will not adhere to the surface of the plastic.
3. A small peice of pipe may work, but only temporarily. Shoving a small peice of pipe in the nozzle will restrict your water flow, and build up pressure around that area. The added metal also will cause the already brittle plastic to fracture, and no amount of glue/sealant will stop water from leaking out. The pressure will just extend the fault beyond the "patch".
4. If you have a nub nozzle left, and a steam leak, take your radiator out (four clips on mine) and flip it upside down. You won't be able to close your hood fully, but enough to "latch" it. This places your leak on the low pressure side, and your water leak will be minimized, (but constant) will generally not leak steam, and will slow down a crack from spreading.
But get a replacement/in service ASAP. Temporary fixes are just that, trying to rig something usually costs more in the long run.