(Author here) There used to be a GitHub repo - but it has subsequently been removed.
Normally, I'd be comfortable sharing snippets under the UK's copyright "Fair Dealing" exemption - but my changes are so tightly integrated into the original code that it doesn't seem right.
Additionally, as I say, my changes are fairly trivial. Read a JSON file, scale an image, stitch it together. It's the sort of thing you can throw together in an afternoon.
As for how much you need to change for it to be reasonable to share, it does rather become a "Ship of Theseus" problem. You'll need to be guided by your own ethics and the laws of your country.
It used to have a link to the full codebase, but the author removed it, perhaps due to legal concerns.