Blueway Software Works (who purchased the intellectual property rights to PC/GEOS from the estate of Frank Fischer of Breadbox when he passed away) seems to have published the source code for PC/GEOS on their GitHub repository.
This repository is the official place to hold all the source code around the PC/GEOS graphical user interface and its sophisticated applications. It is the source to build SDK and release version of PC/GEOS. It is the place to collaborate on further developments.
While I can’t ascertain the exact version that they have published, it looks like it contains all of the UI options/themes provided in Ensemble 2.0 onwards (i.e. Motif, OS/2 2.0 PM and Windows 95). It also looks like they have published the source code for all of the Breadbox titles (extra games and so on) that were sold separately. By the looks of it, they are also re-factoring the source to allow it to be compiled on Windows and Linux with more modern toolchains.