Be warned: It looks pretty simple, but might not work. Don’t blame me if this destroys your MiaB installation
I think you can simply generate a new ed25519 key pair, and replace the files /root/.ssh/id_rsa_miab and /root/.ssh/id_rsa_miab.pub You must use these filenames, because the MiaB code depends on it.
Obviously use the public key to update to rsync target.
Update: ok, I found it interesting enough to try it out myself, and for me the above procedure works.
So I was able to change the contents of that to an ed25519 fine but I think I gave up too early. One of the things I was really hoping for is being able to set something in the .ssh/config for the root user because I really wanted to use my SSH CA instead of depositing pubkeys.