Just replace it with a good used one. Don't dink around with aftermarket electric fans, it's almost guaranteed they won't fit right, and guaranteed to look cobbled on. The factory fan looks, well, factory. I just swapped a used one on to my car, it works great. Found a mint one at Pick-n-Pull for ~$20.
Two other things to remember for W123 chassis cars:
1- The fan does NOT come on when the engine is hot. There is no connection whatsoever to engine temperature.
2- The fan ONLY comes on when the A/C refrigerant pressure (high side) reaches the set point of the pressure switch. So when the A/C is on, the fan may or may not run, this is NORMAL. It is more likely to be on when hot outside and with a full charge. It is more likely to be off if you either have a low charge or in cooler ambient temps.
Silver Star motors in central CA sells the fans for ~$50 with a 6-mo warranty, and there's a guy on the east coast on the MBZ diesel list that will sell them even cheaper (no wty tho.) I thnk new ones are ~$100+.
Regards,