I think this is really cool. It would fit quite nicely on the snes (and would probably have made the whole "all-stars" thing worth while, had this been the art then).
I think that the hills in the middle ground are inconsistent; some of the hills are black outlined, others are not (btw, could use some cleaning on those lines, though I'm sure you realize that already).
I can see where the clouds loop, and that's usually not a good thing...you might want to think about fixing that...(and again, the small cloud could use some line touch up).
Other than some cleaning up here and there, this is a great piece. I really like how Italian Mario looks, and I prefer the new version to the old. This would be ffffing amazing if you actually re-did the whole game...I'm sure there'd be no trouble making it playable. Either way, I'm look forward to an underground portion redone.