Hey, this looks pretty nice as I can tell. Good work with that. One downfall as I can see is the Boomerang Power Up Item and the Boomerang itself. They contain 1x1 Pixels. Usually it is very commom to use 2x2 Pixels in order to fit the whole SMBX's default graphics and so on. You can also use Toad's Boomerang and mix with the colors a bit, just my thought.