Wow. Wow. Wow. 6 stars out of 5. Best hack I've ever played.
Difficulty is vanilla... in terms of this hack picking up on the difficulty curve where SM left off. ie. if you barely made it through SM, you probably won't make it through this. But you don't have to be an all-star to beat it by any stretch of the imagination, either. I can see how if you're a speedrunner or high-level random player, this might be too easy for you, but for the average Joe that can't do tech but no longer has trouble with the original, this is exactly your jam.
I had to up my game with a couple bosses and learn how to fight them properly... simply tanking/spamming won't cut it. But all you really need is mid-air morph, wall jump, and patience. And technically, you don't even need wall jump.
I won't waste your time saying what's already been said about the hack itself (other than beautiful!), I'll just add don't use savestates. This hack uses mechanics that you won't find in the original, so you'll miss stuff and plot points if you do.