The idle adjustment knob is very easily retrofitted.

The hole is in the dash, you just have to punch it out, the idle bracket bolts up where the cruise and throttle linkage bolts up to.

EDIT: the cable actually connects to the linkage in the little hole right near where your temporary fix bolt is.

I found one on a 240d in a junkyard and put it on my grey 300d. fits and works fine.

But the bracket fix looks simple enough and doesn't require finding all the parts for a idle knob.

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion