Mario Builder 64 is a level editor realized fully in Super Mario 64 itself, which should run on real hardware I think. It is intuitive to use and the community created ton of custom levels. I think a custom software is needed to handle the community stuff, but the Romhack itself is playable on an emulator if you want test building your own levels.
The download page for the patch file (remember its not a Rom, its just a patch) got hit by a DMCA. Usually Nintendo does not do that with Romhacks. Sure the patch files itself are not Rom files, but they might contain data that is copyrighted. That's why Nintendo might be annoyed by this.
Get your patch file copies (.bps format) and archive it if you care.
- original download page (showing unavailable content): https://romhacking.com/hack/mario-builder-64
- source code and release build available on Github (but only v1.0): https://github.com/rovertronic/Mario-Builder-64 and https://github.com/rovertronic/Mario-Builder-64/releases/tag/1.0
- version 1.1 and 1.0 of the patch file available at: https://gamebanana.com/mods/514361
- find custom levels at (I think you need xxx): https://levelsharesquare.com/MB64/levels
- documentation: https://rentry.co/MB64-Setup-Guide
Funny enough its still available on Github (sadly only version 1.0 is there, not 1.1, I don't know why). The DMCA hit a fan community hosting Romhacks (only patch files).
Oh that us strange. I just temember what happened to the switch emulation community. They all went off github after the initial takedowns and now the just dont develop on github.
Well Switch emulation is a different story. Especially because it was current gen and so on. But Mario 64 Romhacks and other game Romhacks from Nintendo (including reverse engineering code) usually is not touched by Nintendo.
Mario Maker 3 incoming, then?