From what I have seen in my experimentation with in the new model format, it seems to be substantially different from the prior format.
In order to even think
about rewriting source code, one has to know what the new format is. Since the old format was not described in a public document, it had to be reverse engineered to be understood, no doubt through quite a few hours of testing and trial-and-error experimentation.
I know that Eiz has spent some not-insignificant time trying to figure it out as well, and, given that I haven't seen any announcements regarding breakthoughs, is likely still either busy with it or has given up. (Though I would expect the former, coming from a person who figured out the dxt1 and 5 formats were used in the texture files
That said, I think that it would be easier to write a decompiler from scratch with a known model format that it would be to rewrite existing code to match an unknown format that has yet to be understood. It really is that hard. (Sorry!)