While Nintendo have been sending copyright notices to sites hosting the really rather good Mario 64 port that has been doing the rounds, that hasn't stopped either its spread or the modders who've begun tinkering with it.
First of all, there's Armando Arredondo's HD Mario mod, which takes a tweaked version of the character's model from Super Mario Galaxy, then drops him directly into the 1996 game. It looks pretty good, and while the HD character being placed in a low-poly world might seem jarring, it's certainly no weirder than him being surrounded by humans with normal proportions in Mario Odyssey.
And if you want the textures to match him, CrashCrew are currently working on an AI upscaling. Right now you can see how Peach's Castle looks with HD textures, and it'll be fascinating to see the rest of the game given the same treatment.
There are also mods to enable cheats, disable draw distance, and remove the Nintendo 64's distinctive linear filtering, which leaves it looking like a port for the original PlayStation.
All of these videos come from UnrealYouTube and no, that's not an official Unreal Engine channel.