I don’t think it’s illegal if you’re just teaching your self and. As long as you’re not making any money off of it then it should be cool.
For retrieving models you can 3D ripper DX. Seeing as it’s a Wii game you will probably need an emulator to run the game and then extract the model. I’m not sure if 3D ripper DX has that capability with emulators.
If you want to try and make the same physics like gravity in the middle of a sphere there is a wealth of info on this.
Google this: “ue4 mario galaxy gravity” - So so so so much information on this though you might need to piece together info from other sources to implement this in to your own project.
Also in regards to 3D ripper DX. I’m not sure if it’s legal to rip models from games, Even if you’re not making money from it. So you should probably check. I mean if your only learning or playing around then I don’t think it should matter. But it’s probably best to check.