The pedals from any W123 will work; the M102 engine, however, has the starter on the left side; all the diesels had the earlier right side location. As a consequence, the transmission would have to be sourced from a W201 or W124. Shift linkage from a 201 or 124 may bolt in, or can be adapted. Driveline probably custom length, crossmember by ingenuity.

Re: gear ratios. The gas and diesel ratios for the earlier iron 4-speeds are effectively the same.