Download this if you want, if fine if you don't though, it was lazily and poorly made.
In this pack there's (mostly) 8-bit versions of hopefully all the non-hardcoded assets for the 1.3 version of SMBX.
There are two downloads for this pack, a full version with many different tile & BGO recolors, NPC variations, playables, backgrounds and more, & a smaller version with just one of every basegame file for you to copy and paste into a level with greater ease since the full one has a ton of folders.
some of the graphics don't conform to the 3 color + transparency rule of the NES so that they would look better but if they bother you enough you can just recolor them, I don't mind if you do.
. Note: some screenshots are altered digitally so that either multiple playable characters or multiple block variations can appear in the same screenshot.