I wouldn't plug it since that coolant flow must be needed. It is a small tube so maybe it is just to help sweep out trapped air. You would expect some get clogged by rust, so perhaps wouldn't be fatal, but google it before doing that.

It sounds like you buggered up the hole in the head. Unlikely the bolt was welded to the head, rather it probably corroded in the hole. If you can't get the M-B banjo bolt, similar ones are used on some brake calipers. There are also banjo fittings on your M-B fuel supply to the IP. To fix a buggered hole, look at heli-coil. A bit pricey and I have only used for SAE bolts, but should be metric ones today. I think there are 3 different pitches for metric bolts (coarse, fine, extra-fine), so measure carefully.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans