Someone converted a W123 240D to electric engine shutoff rather than mess with a faulty key switch. It's a matter of finding a VSV that blocks airflow when energized. The overboost VSV from any 80s MB turbodiesel should work. When energized, the VSV plugs one end and vents the other.

Assuming you have a vacuum source in the engine bay, hook up the VSV between the vacuum source and shut-off actuator on the 81 240D IP with the plugged port to the source and the vented port to the shut-off actuator. Wire the VSV so it's energized when the key is in the RUN position. When you turn the key to RUN, the VSV plugs the vacuum source and vents the shut-off actuator line. When you turn the key to OFF, the VSV connects the vacuum source to the shut-off actuator to stop the engine.

I don't remember who came up with this so I don't know his experience with the longevity of an overboost VSV used this way.

Sixto
87 300D