this is a really good port!! there's no errors caused by custom characters when using caps, and i've never gotten any errors when playing the hack. i'd say this is pretty faithful! there is an issue with leaving levels via pausing and selecting exit course, where it'll send you right back into the pipe entrance. this is fixed with exit to castle, so thankfully it's only a minor inconvenience.
i haven't played the original, but having played this with friends, i enjoy most of the levels pretty well. the only ones that were genuinely hard and or frustrating were the bowser level (bowser's colorful course) and pink pipe peaks (PPP). PPP has two stars that require some tricks, one that requires vanilla mario 64 movement to preform the backwards long jump into a part of the level (so no moveset mods), and bomb clip, which if you're not a mario 64 expert, consider it impossible. colorful course is incredibly long and a bit confusing to navigate in order to collect every star. some parts also just feel a bit unfair.
overall, pretty solid! a good time if you want something new.