MiKTeX can be built on Windows and a subset of MiKTeX can be built on other platforms as well.

Getting MiKTeX Source Code

GitHub

MiKTeX source code is available on GitHub.

Tarball

If you prefer a compressed tar archive, you can download it here:

Date: 1/28/2017
File name: miktex-2.9-2017-01-27.tar.xz
Size: 24 MB
Download