It is this type of bus:





MB made van versions of the OM615 and OM616, but they had different manifolds (more upright), starters, valve covers with the filler cap and dipstick at the front etc. You can also modify a "normal" OM615 or OM616. In the picture below you can see a OM615-220D bored to 240D with van manifold:



I doubt that there is enough space for a OM617, but who knows.

You probably need engines from the W115, as W123 engines have the oil filter housing near cylinder four, which doesn't fit.

To get the linkage to work smoothly, you probably need an engine/IP with a pneumatic governor.