Am I missing something? There only seems to be one patch against gtk2 (https://github.com/valerioa/Inkscape-MacOS-Curated-Build/tree/master/valerio-patches) which is related to copy & pasting / the cliboard.1. Most of the modifications are forks of Gtk and some other libraries which are bundled into the distribution. For example to get the menu in the top bar has a patch against Gtk2.
That's obviously true, but if there are not more changes I overlooked it shouldn't require too much effort. For gtk3 it's also much easier to upstream patches as there is active development and frequent releases (so there's a good chance to get patches accepted before the first release of an Inkscape version that actually depends on gtk3).2. The modifications are related to Gtk2, a new rebase would and /should/ start from scratch. Simple because Gtk3 is going to be very different. So both the Gtk fork and the mods to Inkscape need to be re- done. Although I recommend using the ideas from this branch.
Most of the code seems properly ifdef'd already (WITH_CARBON_INTEGRATION).3. Modifications were specific to the mac build. There's some modifications that we could take so long as we had good ifdefs for a Mac build. Things like folder locations and some other bits and bobs.
Well, the fork seems to be the result of the author trying to get the changes upstreamed but not succeeding, see Readme.md of the repository. I wasn't around back then, so I'm not sure if there was any serious effort. Either way the end result is now one outdated and likely dead fork, so I have a feeling actually incorporating the changes into the official repository might be a more solid foundation for future Mac releases (even if it might be a bit more work initially).Overall I'm very impressed by the curated build, but I also understand exactly why this wasn't merged in. I'd probably support a new fork of Inkscape to replicate this work if it means getting a MacOSX release as high quality as this one for 1.0.