I just realized that while this project page links to the repository with the plain python portion of the library, the portion that is written in C is harder to find. So here it is: https://github.com/deshipu/micropython/tree/stage
The branch is a little bit dated, since I didn't upgrade it in a while, but if you look at the three last commits, you will see all the required code. It's not much, just enough to make the plain python version fast enough. Enjoy!