Looks like one would need to replace the double-cam on the injection timer with a drive-plate. Is this part even availible separately, for a price mere mortals can afford? Never seen a stretch W123 in the Pick-N-Pull.
I pulled a belt-driven vacuum pump off an '83 Oldsmobile V8 several years ago to experiment with. Was thinking about fabricating a mount to replace the existing vacuum pump and hanging the Olds pump there, to be driven off the power-steering belt.
But before starting this retro-fit project, fate intervened and I wound up with a new OE pump for my '84 Euro TD and a replacement lever-bearing assy for my '82 240D.
Besides, today new '80s GM vacuum pumps might be as hard to find and/or as expensive as a Mercedes double-pump & drive-plate.

Happy Motoring, Mark

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DrDKW