With git, you can replace diff with a custom diff tool, like difftastic using a global git config file.
Is there a way to do the same with grep? Can you have a different grep tool for git, like ripgrep or ugrep?
It's hard to find anything since most search results show how to use grep. When I use replace it shows how to replace text inside code not how to replace grep itself.
No, it's not possible to replace the grep functionality in git grep because there's no actual call to the grep binary itself. git grep is designed to internally read the relevant objects from the object store so it can go back in history, and it doesn't invoke other binaries to do that, which would make it very slow.
You could certainly try to write your own tool which called out to a Git library, such as libgit2, to implement a similar functionality, but what you want to do with git grep is not possible. Note also that libgit2 doesn't handle SHA-256 repositories, so using it to implement your own tool would leave it stuck on the older SHA-1 kind.