I asked a friend who is a MB service manager in our area about this...he said that the best engine to transplant into a 113 car would be a newer aluminum block V8, but that transmission size would be a problem. So, you would need to adapt the older transmission to the new engine, or rework the tunnel. (this is also the problem with putting the 6.3 engine into any car of that era other than a 6.3 or 600- see Karl Middlehauve's site, M100 Grand). The 3.5 engine would fit a lot better; how the weight compares I don't know but at least you would get to reuse your gearbox, etc. Plus 3.5 could be ordered in Europe with manual gearbox, so the bits do exist if you have a stickshift car.
Personally I would be happy if my 113 just had a little more pep; I asked Gernold Nisius if the Euro cam and higher-compression pistons would make a difference, and he didn't think so. I am going to try a solid-state ignition and tuneup, we'll se if that helps.
Also of interest on the M100 site- Karl retrofit a complete electronic closed-loop FI system with SS ignition and emgine management system to a 6.3 motor. The smoothness and power were amazing, according to a few folks I know who have seen the car. Also, the emissions went down, the fuel economy went up, etc. Mechanical FI was great for its time but clearly can't compete with a modern 'intelligent' system with multiple feedback elements. Of course, you do give up all that cad-plated tubing and arguably some of the most beautiful plumbing ever put on automobile engines, when you dispense with mechanical FI. The only thing prettier is that chorus of vertical Webers on a GT40 or Cobra engine...