Several Legendary Pokemon are still available as well as updated stats, moves, and types. Despite just being based on FireRed, there are some Pokemon from Gen 2 and 3 that will appear in the game early on. Once again, we go back to the fact that the Pokemon sprites are altered with original Moemon sprites while maintaining some characteristics of the Pokemon that they are replacing.
Basically, if you already know the story when it comes to FireRed, you already know where it is going and what happens. The thing about these games is that there are numerous versions using the Moemon sprites as a substitute for the Pokemon. The story is pretty much the same as FireRed.
Pokemon Mega Moemon FireRed Details and More Story Pokemon Mega Moemon FireRed Details and More.