itch.io

Fun fact, the engine can take any Unity scene and make it into a level provided there's a start point. Less fun fact, I have less than no idea how to make that into a proper editor. But in theory, I could try to have the game generate a level on-the-fly from being fed an FBX file (something I already have the Unity editor do via script), and there are in fact model rips of the JSRF levels out there. Still, that's way beyond my skills at the moment.

edit: Just checked, there's no way to get a model file at runtime. Scenes might be another matter entirely, but that takes AssetBundles, which I don't know the first thing about.