You could print to PDF and use Adobe Acrobat to edit the text – however, make sure you delete the certificate file after editing and printing, since it's not a good idea for it to be floating around in an editable format (someone could hypothetically forge a certificate to prove membership).
As mentioned by eblood66, it's best if you can have it signed by the ward where he was baptized.
Another option would be to have the other ward record the baptism, print and sign the certificate, and mail it to you. It wouldn't be a bad idea to work with the other ward either way, in case it brings to their attention other baptisms that may not have been recorded.
Samuel Bradshaw • If you desire to serve God, you are called to the work.