There's not much they can do. If it's all reverse engineered and requires as copy of the original rom for assets, the legal precedent is pretty much set. It's why the Mario 64 PC port is still up.
It's already released anyways. You'll be able to get it even if they force it off line. AM2R is a great example of this. It was forced offline immediately by Nintendo. It's super easy to get today, and even has a launcher that'll dl new updates for you.