Why not ? .
I remember a couple OM617 powered R107's, sound good to me, I don't have the fabrication skills tho'
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My big brother has a 1979 Dodge D200 crew cab long service bed truck, it's very stout but the 360CID V8 hoovers up gas like it's free .
He bought a complete 6 cylinder OM603 and tranny for it but I have no idea who could do the conversion....
In general when I do an engine swap I use a same brand one, maybe larger, maybe smaller (old VW's should retain their 1192C.C. engines if you want to drive and enjoy them as I do) , it depends on the use you'll put it to .