The OP doesn't indicate the scope of the project, but the main clue I took was "written in old [language/dialect] using old [libs]" which to me are the primary drivers of a re-write. In fact, most projects I've been on of any significant market longevity eventually realize that accomodating updates to compilers/interpreters/operating-systems is an important task in maintaining the code base.

In short, I would plan the re-write in phases, prioritizing first the update in libs. It might be that along the way, you can sneak in other optimizations, but the fact that you plan to defer some changes to a later phase can be a great way to stay on schedule and avoid getting "stuck."