Hacking the NES graphics into the GBA game is probably more doable.
I remember reading the FC didn't have enough room to get all the text the devs wanted in Japanese into the game, letting alone adding in the text and additional gameplay of a modern expansion.
Also note that FF2 is already using the maximum ROM size its mapper waz designed for. You might be able to get as far as doubling it if you want to hack it to a slightly different mapper.
But, just being honest, the dufficulty level on that is the "if you have to ask, you probably can't do it" advanced level.