There currently isn't a supported way to backup cPanel DNSONLY servers (the backup feature is not in WHM for DNSONLY servers). If you have a larger cPanel DNS cluster setup re-joining all the servers to the cluster and syncing up DNS could take a while. It would be much quicker if you could just restore the DNSONLY server.
Even if cPanel does not build this feature in, simply documenting what needs to be backed up would be a great first step.
This would also aid in migrating DNSONLY servers when the OS goes EOL.