New radiator time
Even if it were possible, I wouldn't mess around with a repair. Spring for a new radiator (about $400). Mine blew out a big hole in the plastic side tank this summer, fortunately while in my driveway. I had always used MB coolant, with regular flushes. I think it's the nature of the beast, and MAYBE the M119 engine is more likely to have radiator problems since underhood temps are higher than on the 300E (my theory). In any even, the plastic on the side tanks gets brittle, so do yourself a favor and replace it now. You do not want to fry this fine engine.