Try the bottom option on the portal site and get the license emailed to you again. That's the only way I can think of that Fast Spring might have the keys in its system (just in case they inherited something when eSellerate shut down).
The searching of old threads doesn't portend good news for old eSellerate licenses though. Here is an example.
https://forums.ootpdevelopments.com/...d.php?t=342442