Make it easy on them. Send them each a letter with a self addressed stamped envelope including a letter stating something like "Effective (the date you got him back) I hereby transfer ownership of the following dog (add the dog's name and registration number and any other info like microchip, sex, color, etc) to yourself, then another line "I am unable to find the AKC registration certificate for this dog." Add a line for their signature including contact information, so they can just sign and put it in the mail.

Then the AKC case management might be able to manage the transfer of ownership.

Thank you for all the wonderful responses, I'm doing a blend of all the suggestions. I right away scheduled X-rays for OFA and CERF exam, I e-mailed and left phone messages for both parties that they could potentially still be liable for anything the dog does and for his care - they responded right away and said they've never received new AKC certificate which I ordered for them to sign; SO I told them that I would send out a letter for them to sign with a SASE which they both agreed to sign and send back:) I'll just keep praying until it's all done :)

Thanks so very much for all the useful suggestions!