Open the back up copy. Perform a find that finds just that one record and leave your backup copy open.
Go to your regular file and use import records to import records from the table where you have just found your one record.
Import records from an open FileMaker file imports the current found set so you only need to produce a found set of just that one record for this to work.
(To preserve the integrity of the original record I'd suggest you consider leaving unticked the option to 'auto-complete by lookup... etc'.)