Since Gen3 on the GBA pokemon never stored the pokemon files in plaintext in memory, they are always encrypted to prevent direct memory editing.i was looking at a pokemon edit code to replace or have xxx pokemon in your slot or bank but i need to find the proper memory for it.
im only able to find at the moment the memory that changes the pokemon in your preview but not the actual pokemon you have, example you goto your pokemon and it shows it, the memory i found changes the preview of the pokemon to what ever you want, so magicarp will be there and previews as bulbasaur for example
To find them, dump the encrypted pkm file with pkhex and then search for the encrypted values.