Are you familiar with avisynth scripts?
If you extract the video file from the DVD to a single mpeg2 file, and then use for example Megui's file indexer, this script will make a quite good upscale:
Mpeg2Source("DVD.d2v")
nnedi2_rpow2(rfactor=2, cshift="spline64resize", fwidth=1280, fheight=720)
(You will need the avisynth plugin nnedi2 to do this)
You can also try getting nnedi3 and replacing nnedi2 with that in the script and see which result looks best.