Okay, using the English font from the DS version is too big. Well, it could fit but it takes up about half the space of the GBA screen and people don't like it even know Battle Megaman Network GBA uses a similar size font and box size on the GBA.
Anyway, I found increasing the text box by one 8 x 8 square and using a size 10 font, it looks rather nice.
Note: I just made the image quickly so it's only to show a rough idea what it would look like. Custom is on the left, original on right.
I think a better 1bpp font needs to be used or a custom one needs to be made. What do you think of this idea cocomonk22? Hacking it is going to be fun and difficult know.
EDIT: The Dotum font works really well. Here's a better mock up of the image:
The blue and yellow squares on the left hand side are just to give me an idea how to hack the font in. The GBA uses 8 x 8 squares to display graphics and the font is going to use two squares. The first step is to make the GBA load the OAM graphics as two 8 x 16 (instead of 16 x 16) squares on three lines.
The bobbing that appears at the bottom of the screen in the GBA version will have to appear on the far right, like in Phoenix Wright which can easily be hacked into the GBA version.