Is it legal to receive donations for a CS?

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IMO, except if you show some people a grunt model from the original game, and say something like: "Like what you see? Donate to us!" That's illegal.

Strictly speaking they're still not forcing anyone to give money. It's still out of their own free will.

Actually if they just repackaged the original Amnesia or misled people into thinking it would be a total conversion mod and just released some crappy CS I'm pretty sure they could get sued for false advertisement or bait-and-switching even though no purchases were made.