so um
if you click on "Show Sprites" in advancemap (the brendan sprite in the top bar), you'll see that there is actually an "invisible" overworld in the top of the northernmost tree
it's pic is currently 240, which places it in a dynamic ow section thing in the ow table
this makes it a little weird, you probably should be able to just change it to mew (140) and then make it have the hidden movement and it should be fine overworld-wise.
inside the script, one can just replace every occurence of "0x19A" (Deoxys' index) with whatever Mew's index is (0x97, I think ahahaha). this takes care of the battle and the cry that is played
but yeah this all works perfectly fine
it might also be in your interest to go to address 1D402 in a Hex Editor and change the "97" to "00" so that the mew will actually obey you
also go to 1D3F0 and change the first 4 bytes there to simply 00 00 00 00
this makes the deoxys obey, but that might not be a thing that you really care about ahaha
good luck m8~
