I took a brief look, and on the surface it doesn’t look too bad - a lot of the changes would need to occur in Z.Botan.FFI but aside from a few things like the CBytes type, most of the Z-stuff used here appears to be a lot of type-aliases and re-exports.
More concerning is the issue that it relies on an out of date fork of botan pulled in via git submodule using a Custom build-type script - it is 3 years out of date, and would need to be replaced with a maintained dependency.
I may not be the best candidate for the job, but I have written my own bindings to libsodium before, back before cryptonite existed (and saltine didn’t provide the APIs I needed), so if time permits, I may give it a go. No promises though 