Making a Rom hack

Is it possible to make a rom hack of this game, I for one would like to see a completly new story, enemies and items.

I think It's possible but I have no idea what is used to hack into ROMs, I've seen so many hacked Sonic 1 games, Sonic Megamix being the best one.

I reakon there is a Sega Genesis hacking tool somewhere but I've no idea where to find one.