A nice work
I don't have word about the work you had done, being a old gamer, this game makes the child inside me flourish, with extra characters!
But i do have a complain. I have been hardcore with this game since i was 4yo, the colision detection could be made smooth and enemy spawn are not fixed. I noticed this on the 8-2 level, where i *used* to hide on edge of the 1st stair and the flying koopa past without touching me. I noticed the spawn point was kinda random in relationship to the velocity you went (I was mario), as i died pretty much always on that spot (and never on the original game, 18 years of experience on this SMB!). Another little bug is that mario sometimes fall down, even running, on the 1 quare ledges
But overall you did an amazing job, and hope to see more games from you!