You can shoehorn a 603 engine in there... one of the Finns did that in a green 123 (there's videos of it around here somewhere). Or maybe it was a 606, either way, it's possible if you're willing to expend the effort. If you just want more power, intercool the existing 617 and have the pump tweaked by Myna Diesel.
The W124/603 combo has a 722.3xx tranny with a gear drive speedo. Sadly, only the V8 engines in the W124 had electronic speedos... this makes it awful when changing gear ratios, or dealing with a wobbly needle that refuses to go away even with a new, properly routed cable.
