Couldn't you possibly stick an OM617 into the van and also hook up an electric cooling fan instead of the stock engine-driven one? That would give you enough room to fit the extra cylinder.
As for the oil filter, relocating that isn't hard at all.
Those vans are fantastic.

Phil Forrest

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1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson