It isn't the plastic expansion tank next to the strut. It is the plastic tank that runs down the left (Driver) side of the aluminum raditor core. I didn't know if that plastic part is sold separately or if as FAD suggested, I should replace the entire radiator. It seems a shame to discard the whole rad because of an inch crack in a plastic componet. I'm still using the 42-year-old Behr radiator on my 190 SL. A new OEM radiator is around $300.
Fad ... you suggested an upgraded radiator. Do you have a particular brand in mind?