I've been following this thread for a while, and even attempted to modify the game code myself but was unable to. I remembered this existed though, so I decided to take another shot at it. I used this version of the Undertale Mod Tool:
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I'm not an expert at this, and I have not yet tried to make changes and recompile things, but this version of the tool DOES seem to show everything important, including text and the different sprites being used. Here's the bad news: Nearly everything involved in the animation of the character has the censorship baked in. It's not a different object or a different later, it's in the image data itself. Short of redrawing it, it's not going to be possible to uncensor this game. The author was very careful.
I'll keep working on the text part and see if I can get something working for that, and maybe export the image to see if anyone wants to try to uncensor it by hand.
EDIT:
I've played around with it a bit, and I have the beginnings of something functional.
Obviously it needs some work, and I need to reverse engineer how some text sizes are decided, but at least have enough of an understanding of the script to modify it. It won't be perfect, I'm using a translator, but the dialog is simple enough that I should have something workable eventually.
In the meanwhile, someone who is better at art than I am should take a crack at uncensoring these two images. Please be careful and don't modify anything besides the dick and pussy, and make sure you download the full sized image. Otherwise it won't load correctly.
I'll report back when I've made further progress.