There's a lot you can do for yourself that's not illegal, but can quickly BECOME illegal as soon as you try to profit from it.
If you were to cast your own suit off of a statue, then as long as you don't sell the suit, you aren't breaking the law.
It's only an ethics question if you intend to use it for anything other than a personal costume, or claim to have sculpted the molds yourself. If you say "This is my costume which I made from molds of the statue I own." then you aren't doing anything ethically wrong, in my opinion. Of course if you, instead, choose not to reveal the source of the molds, you are then basically lying about it, which is considered unethical by most.
Of course other people's opinions will vary, but the reality is that as long as you're not trying to hide the fact that somone else did it, or otherwise trying to claim someone elses art as your own, then what have you done wrong?