A few years back, someone created a hack of SMB1 called 'Luigi and the New Quest'. It's got improved graphics and all-new levels that are really tough.
If you've played it before, how far did you manage to get? It's been a while since I played it, but I'm pretty sure I made either to 3-4 or into world 4 somewhere. If you haven't played it, go play it! If you try it on Nestopia though, you'll get some strange bugs that make it unplayable (including a koopa castle that's entirely underwater).