It might be compressed, it might be differently encoded (i.e. "Letter 49" inside the game's font isn't what your computer's Letter 49 is).
It's not as simple as "open with hex editor, see text you can change" in most cases, especially for games predating the PSOne era.
EDIT: A few quick Googles reveals that it's a hack of Decap Attack which changed around some sprites. Perhaps that should be your starting point, figuring out what changed from the original game?