Is the earning considered good if a certificate of experience was forged, and it was later found that its owner fulfills all the required conditions to obtain this certificate, given the inability to replace the forged certificate with a real one?

As long as you have met the required conditions for the work, there is no sin upon you in performing it as required. Seek Allah's forgiveness for what has passed from you, for "remorse is repentance," just as repentance from a sin is "remorse and seeking forgiveness."