@doc Having backups would have saved you some pain, but you can rebuild it.
Understand this memory is vague, but I seem to recall if the fog client is installed on your target computers, when the fog client checks in, the registration will be created, but in a pending state, where you need to authorize the client in the FOG Web ui. If it still works that way, this may be a way to rebuild the registrations without much pain. The other options may be to add the hosts via a csv file into the host table, there may be a web ui option to import hosts too. You will at least need hostname and mac address.