Failure of this plastic elbow is fairly common with age. Often the broken pieces will fall into the radiator anyway (as was the case with my father's '93 E320). I am not aware of the small plastic pieces ever causing any problems (others may wish to comment), nor am I aware of anyone trying to retrieve these pieces once they have fallen in. I would be inclined to suggest that if you cannot pull the broken piece out, then I would push it in. I would advise against trying any mod such as plugging it permanently. The designers put these things there for a reason.

__________________
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.