You can contribute by reporting bugs and requesting features on the issue page.
However, please see known issues before reporting bugs.
You are welcome to leave pull requests to add new features. However, due to the fact that eryn was made to solve specific problems, it might not be accepted.
This doesn't mean that forking is forbidden. Feel free to fork the project and change the engine in any way that you like. If you do so, please respect the license :)
Yes, the code is far from perfect because the base of the engine was written a long time ago. However, it's battle-tested and it works. Refactorings are welcome, as long as the changes aren't too major.