Is it possible to run an archive on it? I have seen where the 2 gig limit was reached, but an archive completed successfully, and then by running the compact utility, got the file below the 2 gig limit. Hopefully this would work on a 20 gig file.
I would also suggest that before you continue on, that you make a backup copy of the pst file so you have it in case something you try corrupts it worse.
I did some quick checking and it does not appear that the old crop pst tool from MS is listed as working for a 2007 20 gig file. The old tool would crop some portion of the oversized file, you could not choose what, to get the file below the size limit, so that the scanpst tool could work.
MS article about pst repairs
Pay Recovery software