I was told that it was possible to edit the checksum in a Ruby or Sapphire save file using a save editor to cause the Colosseum Bonus Disc to give me a shiny Jirachi (or any other Jirachi of my choice out of the possible 65536).  Is this something that can be done with PKHex?  I opened up a save file with it and I don't see an option to edit the checksum (only an option to verify it). Also, the same source told me that I may need to edit other variables so that my checksum remains valid, or