Honestly, I don't know? I suppose it might be easier for me in unity, since I have more experience with it, and the editor is a bit cleaner.Interesting. Since you want to rebuild the whole thing, is there any reason to still go for Unity and not renpy ? I guess there's a reason the Dev went with Unity but I really don't get it, all the game's functionalities seem easily doable in renpy.
Also, the you can rip the assets out of the game yourself or use the final scenes that are in the HQ assets packs, it's mostly up to date. Also the HQ assets packs are higher quality, that's what I use when I do character edits for fun.
If you do rebuild the game, it would be nice to add a way for us to mod it. I don't know how that would work in Unity but it would be greatly appreciated.
As for mod support...
Well I'm not sure how easy it would be to include new images (It's been a while since I messed around with packages or assetbundles, and I don't know precisely how annoying it would be to import anything but a raw textfile - that should be fairly straightforward), but I can see ways of importing new storylines fairly easily - so adding more text with existing images and backgrounds would be fairly simple. I was already considering a way of implementing branching storylines with - admittedly reused - assets and making the story more interesting. If nothing else, I'd like to make it so that the 'one off' sex scenes are a bit more repeatable, and so that you actually get the text in the vault scenes.
I might take a peek at renpy again and see how hard it would be to implement, but I do have experience with unity and I'm not sure how much effort I would want for a proof of concept.