When you go to the 2.1l style case (which some late 85 1.9ls had by the way)
the top of the case where the halves come together is different.  Since the
air box mounts along these case halves this makes the way it mounts somewhat
different.  I know that you could probably modify the mounting brackets with
some work and experimentation, but the much easier route is to find a late
'85 1.9l engine in the junkyard or a 2.1l Vanagon and get the air box.  With
your late 85 throttle switch setup you might try to find a 2.1l Vanagon
Throttle body as well.  These are actually larger in inside diameter and
would give you some more intake air flow while still plugging into your
digijet harness.  Just FYI.

Thanks,
Ken Wilford
John 3:16
http://www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242