JSLinux runs any disk image reachable over the network. You can find the Alpine sources over at https://github.com/alpinelinux. The Fedora sources are over at https://src.fedoraproject.org/. I don't think the script to package the pre-built images is documented somewhere, but it's all standard Linux you can install yourself in a virtual machine if you want.

The Windows 2000 sources are probably leaked somewhere on Github, or you could apply for a job with Microsoft maybe. I managed to get ReactOS to boot in JSLinux once by installing it into a standard QEMU image and splitting the image into chunks as required by the emulator code. You need quite a fast CPU to run a full, modern OS at acceptable speeds, though.