I was working on Ninkuu Gaiden for a while. I probably won't get back to it, so hopefully it's okay if I just dump what I know here. I'm afraid dumping the script would probably be more work than just playing the game and transcribing it.
There's no standard text encoding. Each string has its own set of tiles with only the characters it needs, generally in the order they appear. The tiles are compressed, some with RLE and some with LZSS. I think which it uses depends on what palette color the text uses. The good news there is that the tileset number is part of the string itself, so it should be easy to replace just one or two and make them all use those.
Each character is two bytes - top tile, then bottom tile. The font tiles start at 14, and the first three are (usually?) blank, ゜, and ゛. Since the other tiles are in the order they appear, strings usually start out like 06 14 17 14 18 14 19 14 1A, the first byte indicating the tiles. The only specific address I have written down is 0x5E368, which is the first boss intro (「ふうん・・いさましいペンギンだな。).
FA: Process next bytes with no delay (used in large kanji)
FB: Change character pose
FC: Clear window
FD XX: Wait XX frames
FE: Line break
FF: End of text