Or fork the code. That's the great thing about FOSS.
GitHub in particular has a "fork" button to make it super-easy. So fork, do your edits. If you want, make a pull request with the original project. If they take it, great: the fork can cease to exist.
If they don't want to merge it don't take it personally but go your own merry ways. And this is what didn't happen with actix.