The hack is done and can be downloaded here: https://www.romhacking.net/hacks/6258/
I've seen some graphic improvement hacks for SMB, but I mostly feel like they take away from the game's own style, usually importing graphics from SMB 2 or SMB 3. So I decided to make my own graphics hack, replacing them with SMB2J, that has mostly the same style, just a little bit more polished graphics. I initially thought of simply importing the tiles directly using TLP, but apparently it's not compatible with FDS files, so I'm redrawing all the graphics using the FCEUX PPU viewer as a basis. So far I've done the font, the bricks, the mushrooms and the moving platforms. However I'm having a hard time finding some of the sprites, like the floor blocks for 1-1 and the fire flower. This is only my second hack so maybe I'm just missing something. If anyone could at least point me at the right direction I would appreciate.
NOTE: Considering the amount of hacks available for this game, it's possible that someone has already done this exact same thing, I just wasn't able to find it.