I remember someone from the Mercedes newsgroup (alt.auto.mercedes) installed a GM 4.3 liter V6 engine and transmission from a Chevy Astrovan into his 123 300D.. He had to fabricate motor mounts and do something with the throttle linkage but it worked out. He even had a webpage with photos of his progress.. search for it on groups.google.com.

Why not just find another used 300D Turbo engine and throw it in?