Very important please read.
 

A friend of mine is desperately searching for a usable crankshaft from a 603970-971 or 972 PN: A6030312501 old numbers: A6030300801-A6030300701. They were found in '90 '91 350SDL W126 And S350 turbo diesel W140. Please contact me if you hear of something. e-mail: tr105@simnet.is

Car-Part.com--Used Auto Parts Market

I did a search under 91 Mercedes 350, crankshaft. Shows two available. One is in Philliston MA, the other is in Toledo. One in MA $500 (ouch), one in Toledo says call.

Searching there for an engine, theres about 14, though most are cores/parts motors. Likely one of them would sell a crank.

check in the parts section I think someone has a 3.5L crank for sale