I would sure try to find a good OM602 to drop in. Having owned both a 617 and a 602, I think the latter is a more refined engine. Seems to me it would be much less work to replace with a like engine, too. Have you posted a WTB thread in the parts classifieds? Engines are out there!

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)