I'm new here, and still fairly new to the Mercedes world. But, your project car is very similar to one I am restoring, a 1964 190D. Mine had a fatal engine death by lack of timing chain, haven't opened up the engine yet to see what it looks like, might be repairable with some valve work. I recently got a 1980 240D that was parked due to #1 rod sticking out the side of the block, and that car came with a spare 5 cyl out of a 300D, got that all swapped in and should be road ready this weekend. While I was working on this one, I got to thinking, this 240 engine has a bad bottom end, the 190 has a bad top end....hmm, will they interchange parts? I need to do some measuring with things torn down to know for sure, but at first glance it looks like it would work, other than a few cosmetic casting differences. What I also noticed that may be beneficial to your situation, is that the block casting appears to be the same in the areas of mounts and transmission facings. I would feel good about thinking it could very well work out with the 240 engine for yours, and they are plentiful on the cheap at wrecking yards in rear ended cars and such. I did notice the block bosses were a little different on the lower right front, where the alternator mounts on the 240D, nothing a newer bracket wouldn't fix.
-Chris