A few weeks ago, I have encountered the Uno Platform, "a Universal Windows Platform Bridge that allows UWP-based code (C# and XAML) to run on iOS, Android, macOS, WebAssembly, Linux and Windows 7." It is currently the only feasible solution for getting UWP programs to run outside of Windows platforms, and it is also free/open-source under the Apache 2.0 License.
However, Uno currently does not support Windows OSes older than 7, so ReactOS in its current state is also unsupported. Supported platforms:
- Windows 10 (via the standard UWP Toolkit)
- Windows 7 (via Skia)
- iOS, macOS and Android (via Xamarin)
- WebAssembly through the Mono Wasm SDK
- Linux through Skia