Backporting a script as it would generally been known has been done on various games in the past. It is usually a fairly involved thing though.
Step 1.
Find the script in the NA/EU/whatever version.
Understand how it works as far as placeholders ("it costs ???? to stay here for the evening" sort of thing that varies between locations in the game), pointers, markup and whatever else. Being a GBA game you don't have to worry about space for most things, for older systems this is very much a concern.
Probably want to understand the font as well.
Step 2.
Find the script in the one you want to overwrite, in this case Japanese version.
Understand how it works as far as placeholders, pointers, markup and whatever else.
Most Japanese games do not feature decoding as it would be done in the English versions of a game. This is why you need the font, or abilities to alter the game to speak the original encoding (rather easier said than done, though not impossible -- 8 bit text conversion being a thing in more conventional translations).
Now you get to match up the English script to the Japanese one* and overwrite things using whatever formats the Japanese game understands (be it stock or what you changed it to), tweaking pointers and possibly lengths as you go. Hopefully there is no Japanese bonus content you need to further translate, and the game stays vaguely close to the original script (Nintendo's flagship stuff tends to be reasonable, no samurai pizza cats there) such that you can overwrite it, if you are exceptionally lucky it might even be the same ordering but I would not bet on it happening.
Having necessarily studied the script to make this you will probably also be thoroughly sick of it by the time you come to actually playing it.
*this is perhaps the only time it is acceptable to use machine translation in a final hack as you are trying to confirm the text being replaced matches what it is being replaced with. You might still encounter things that were altered in translation though to be more localised terms, or chime with the standard phrases in the series.
The other alternative is that you figure out what the changes made were from some other comparison review, wiki or whatever is out there and replicate those in the original. Depending upon what was done then this can be easier said than done -- some more stats and a few more items in chests is one thing, full quality of life and extra features is quite another, indeed not wanting to have to replicate those is one of the reasons people backport scripts in the first place.
