Well, I found one 150A alternator for sale on ebay specified to fit a diesel W123, so I asked the seller whether the alternator would fit in the place of mine (gave him the number on the alternator), the seller said it would so I bought it.

It did fit, and now I get 14.1V at idle with headlights and fan on, prevoulsy I had something like 13.2V unless I revved up the engine.