Remember this game suffered some serious DMCA threats a long time ago. And flash player has not been officially supported by adobe for years. It's a lot safer, simpler, and smarter to make a subtle remake of this game from scratch using a different engine and character. Simply add a "hidden" cheatcode that changes the main characters' appearance, the goombas and her power-ups, back to the original Peach once the game is done, and deliver the assets unofficially like a mod.
For building a fanbase, you can even use another popular IP instead that won't get any DMCA issues: Touhou project for example.
Someone could start working on a platformer inspired by MIMPUT with nearly the same gameplay and overworld, using Yukari Yakumo as a protagonist (she's blonde and would work perfectly) and re-using all of the same enemies and scene concepts, but with different colors and shapes, themed after youkai and oni. If someone works on it really out of love for Peach and Nintendo, they just have to make sure each model is easily replaceable with the right assets, without breaking the scenes. Using simple semi-2D, 3D models in Godot or Unity may make this simpler, I don't know. But if you want to put work into a project like this one, don't repeat Ivan Aedler's mistakes regarding law and DMCA.
Ask yourselves the question: What makes MIMPUT great, and still unique to this day? I'd say the platformer's game feel, the power-ups, the core SMB3 heavy inspirations, with a slightly more floaty feel. The level design thought with the Tanuki suit in mind. The worldmap and its levels from 1-1 to to the 8th world, and its general atmosphere. The variable breast size through power-ups. People have rarely tried to recreate this SMB feel in eroge platformer.
As for Touhou and the platformer power-ups, think about it: Ran for the tanuki suit, Mokou for the fireball, Cirno for the pinguin suit, Orin for the catsuit, Meiling for Striker, Suwako for frog, etc. You can basically remake all of MIMPUT's power-ups gameplay wise using Touhou characters, and sexy outfit inspired by each of them. You can use Yuyuko's cakes instead of mushrooms for the breast size changes, it would make sense and you can let them walk and behave like the mushrooms in Mario gameplay wise. And Touhou lets you make creative and fun power-ups: Yukari's portals, Youmu's sword slashes, Reisen's gun, etc.
You have all the chara-design and outfit designs ready, and depending on how you decide to make the game, you can use AI to fill in the gaps. The core game design would be highly similar to a worldwide success, SMB3, and the eroge aspect inspired by one of the oldest western eroge, MIMPUT. This would be an easy artistic success.
TL;DR: I would advise against modding this game, or picking up its development from 3.48. Instead, make a Touhou platformer heavily inspired by MIMPUT and SMB3 with Yukari as a protagonist, and wait until the game is well advanced to drop assets that replace the characters and music after Mario in a very subtle manner. Use a more modern game engine like Unity or Godot, just focus on the SMB3 feel for the gameplay and keep most of MIMPUT's ideas.
Edit: Here are realistic steps you guys can take to remake the game:
Two important things are needed:
1. 2D animations
2. The core coding to make the platformer work in Godot, Unity or another engine.
For the 1, you can use MIMPUT's extracted assets as inspiration for making tilemaps, 2D art, animations etc. Using Yukari instead. It's possible to use AI to modify an existing tilemap, using controlnet, and reproduce the same animation with a different character based on MIMPUT's animations and original assets. Start with an idle animation, walking, running and jumping. Everything else is secondary.
For a pixel-art feel, Aseprite/Libresprite are a great tool, with big communities and lots of resources for leaning 2D art, and 2D animations.
For the 2. There are many tutorials on making 2D platformers with both Unity and Godot. Or directly in C or Python if you have the skills. If someone can make a working and simple level 1-1, with a design highly similar to MIMPUT, inspired by MIMPUT's level 1-1, the hardest hurdle will have been overcome. I believe in a month or two or learning how to use one of these game engines and following simple tutorials, with 1h of work a day on average, it will be possible to make a working level 1-1 with temporary assets. If it feels similar to walk, jump, and move, you won.
With just 2 people, one working on the art and another working on the game development, you can go very far. The assets are here to try, but an artist is probably required to remake scenes and characters with the Touhou IP instead of Nintendo's, for legal reasons.
You can upload code for the game on gitgud.io and work open-source style, collectively on it. And maybe someone will make art for it. For the art, you can share links to art and new assets in this thread, someone may use it to start working on a remake of the game.
For the AI tools for art, Automatic1111 and ComfyUI are a great place to start. There are tutorials for 2D animations using those.
Free assets can be used for sfx and music to begin.
I found this video showing how artists and devs cooperated to make a silksong clone, with almost no communication other than a correct division of the workload ahead of time.
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