I'm using this space to make some notes for myself and possibly provide Google fodder for anyone else in the future that needs it...
The right head part number is 642-010-06-21. This appears to be the bare casting, no valves, no cams, but would have to include the head cover. Interchange, according to MB parts catalog appears to be (all with 3L V6):
2011-2013 E350
2011-2012 GL350
2010-2011 ML350
2010-2012 R350
2010-2017 Sprinter 2500 & 3500
Exhaust valve part number is 642-050-05-27 and there are 12 of them. Interchange is (all with 3L V6):
2007-2009 E320
2011-2013 E350
2007-2009 GL320
2010-2012 GL350
2007-2009 ML320
2010-2011 ML350
2007-2009 R320
2010-2012 R350
2010-2017 Sprinter 2500 & 3500
Intake valve part number is 642-053-04-01 and there are 12 of those as well. Interchange is (all with 3L V6):
2007-2009 E320
2011-2013 E350
2007-2009 GL320
2010-2016 GL350
2007-2009 ML320
2010-2014 ML350
2007-2008 R320
2010-2012 R350
2012-2013 S350
2010-2017 Sprinter 2500 & 3500
Upper spring retainer part number is 668-053-00-25. 24 of those in the engine. Interchange is:
2014-2016 E250 (2.1L L4)
2011-2013 E350 (3L V6)
2007-2009 GL320 (3L V6)
2010-2016 GL350 (3L V6)
2016-2016 GLE300d (2.1L L4)
2013-2015 GLK250 (2.1L L4)
2015-2015 ML250 (2.1L L4)
2007-2009 ML320 (3L V6)
2010-2014 ML350 (3L V6)
2007-2009 R320 (3L V6)
2010-2012 R350 (3L V6)
2012-2013 S350 (3L V6)
2010-2017 Sprinter 2500 & 3500 (3L V6)
Intake valve springs, of which there are 12, has a part number of 668-053-00-20. Interchange is (all with 3L V6):
2007-2009 E320
2011-2013 E350
2007-2009 GL320
2010-2016 GL350
2007-2009 ML320
2010-2014 ML350
2007-2008 R320
2010-2012 R350
2012-2013 S350
2010-2017 Sprinter 2500 & 3500
Exhaust valve springs -- this one is weird. There doesn't appear to be a part number that cross-references to the 2012 GL350. More research needed. Might be an anomly in the catalog since this spring does interchange with every other OM642 in 2012.
Lower spring retainer part number is 668-053-01-25. 24 of those in the engine. Interchange is:
2014-2016 E250 (2.1L L4)
2011-2013 E350 (3L V6)
2007-2009 GL320 (3L V6)
2010-2016 GL350 (3L V6)
2016-2016 GLE300d (2.1L L4)
2013-2015 GLK250 (2.1L L4)
2015-2015 ML250 (2.1L L4)
2007-2009 ML320 (3L V6)
2010-2014 ML350 (3L V6)
2007-2009 R320 (3L V6)
2010-2012 R350 (3L V6)
2012-2013 S350 (3L V6)
2010-2017 Sprinter 2500 & 3500 (3L V6)
Valve keeper is part number 000-053-11-26. It appears to interchange with every engine Mercedes has built in the last 10 years.
Valve seal part number is 000-053-56-58. Also, very large interchange list.
Rocker arms are part number 642-050-01-33. 24 in the engine, interchange is (all 3L V6):
2007-2009 E320
2011-2013 E350
2007-2009 GL320
2010-2016 GL350
2007-2009 ML320
2010-2014 ML350
2007-2009 R320
2010-2012 R350
2012-2013 S350
2010-2017 Sprinter 2500 & 3500
Lifters are part number 642-050-00-80. Pretty much every Merc engine since 2006.
Left intake cam is 642-050-28-01. Interchange, all with the 3L V6, is:
2007-2009 E320
2011-2013 E350
2007-2009 GL320
2010-2016 GL350
2007-2009 ML320
2010-2014 ML350
2007-2009 R320
2010-2012 R350
2012-2013 S350
2010-2017 Sprinter 2500 & 3500
Left exhaust cam is 642-050-35-01. Interchange (all vehicles with the 3L V6):
2007-2009 E320
2011-2013 E350
2007-2009 GL320
2010-2015 GL350
2007-2009 ML320
2010-2014 ML350
2007-2009 R320
2010-2012 R350
2012-2013 S350
2010-2017 Sprinter 2500 & 3500
Here's where it gets a little interesting. Right intake cam is part number 642-050-36-01 and the much-shorter interchange list is:
2007-2009 E320
2007-2009 GL320
2010-2012 GL350
2007-2009 ML320
2010-2011 ML350
2007-2009 R320
2010-2012 R350
Right exhaust cam is 642-050-46-01 and it has the more typical interchange list:
2007-2009 E320
2011-2013 E350
2007-2009 GL320
2010-2015 GL350
2007-2009 ML320
2010-2014 ML350
2007-2009 R320
2010-2012 R350
2012-2013 S350
2010-2017 Sprinter 2500 & 3500
Okay, that is a big dump of stuff, but it tells me that if I re-use my right intake cam, I can drop in a head from a Sprinter from 2010-2017 which widens my search parameters.
What is odd is that in looking around for bottom-end bits, the crankshaft doesn't appear to exist.
The rods are part number 642-030-41-20 and interchange with:
2011-2013 E350
2011-2012 GL350
2011-2011 ML350
2011-2012 R350
2012-2013 S350
2010-2013 Sprinter 2500 & 3500
Also, there's no part number that I can easily find for the engine block itself. So as far as the bottom-end is concerned, I don't have a good idea for what I could use except for a 2012 GL350. My WAG is that the block is going to be pretty much the same across the OM642s, but I'm not sure if there would be a drop-in interchange with, say, a sprinter short block. I know for a fact that the swirl motor actuator (642-150-05-94) does not interchange with the Sprinter or any other of the OM642 engines other than a short year range on the ML and GL, so I'm sure there are other differences. But, if the block is truly trashed and I needed to start over, I could get a used engine for a somewhat reasonable (by Merc standards) sum and cobble it together with a combination of parts.