The m103 in your car is one of mercedes tough'est engines, I would find a low mileage replacement and swap it over.

If you want more go, you could convert it to a manual transmission which will make it faster off the line and more fun to drive.

The only other easy option is to find a later car with the early m104 24valve engine and swap that engine in to yours, along with its wiring harness and ecu's.This is the only engine swap that you can just swap bits over, all other engines need a huge amount of rewireing to work.

Hope this helps

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87 w124 zender 3.6 - 3.0 kompressor in progress!
96 w124 e300 diesel - slowly being modded
98 w208 230 kompressor - m103 testbed SOLD.
96 s124 om606 superturbo diesel 508bhp