The changes to the original source of InterViews and Unidraw have been mostly limited to bug fixes and backward-compatible modifications. This has been tested by building packages like Target Jr and MiXViews on top. The original schem-3.1 from John Vlissides still builds as well, as would probably ibuild and doc.
We've applied all the patches we can find from various InterViews archives, like those of the PDP++ development team. Know of any more you want applied to the ivtools source tree? firstname.lastname@example.org
Here's a piece on our experience to date reusing and evolving InterViews, Unidraw, and idraw.