And that's fine – but my point is, you should not expect it to be so easy to make alterations to the game. If it was easy to change the graphics for the final boss, it would be just as easy to change the rest of the graphics as well and easy for unscrupulous people to try to make a quick buck. It is in Square's best interests if making such alterations is a difficult task.
(To be clear, I have absolutely no affiliation with Square and would point you in the right direction if I had any more ideas on how to proceed. I'm just saying, there are good reasons to expect even making a minor hack for yourself to be enormously difficult in this case.)