hey, alex.
this would be tricky, and while possible, not recommended.
MUCH better to re-master and make a new DVD.
you can drop your existing master into FCP,
chop out the bit you don't want,
export as a reference movie, then do your encoding etc.
if you want to persist with the mpeg file, then i would suggest not even changing it,
but rather doing the edit in DVD studio pro.
(because that's how i know how to do it.. there may well be some way to edit the mpeg)
either way you will be forced into imperfect edits as you can only cut on the I-frames.
nick