Hereby I present you, the Pokémon Mystery Gift Editor!
As the name already suggests, you can edit Mystery Gifts from the 4th and 5th generation games. It also has some extra features like exporting Wonder Card images and generating Action Replay codes.
At the moment there's being worked on a new version. But if you want to use the editor right now, please go to the downloads section.
Keep in mind that I do not support these older versions anymore. So please do not report any bugs