FAViR's source code is all free, licensed under the GPL. It's available in the downloadable package but is also hosted on line. FAViR's source code is hosted by Savannah, a free software project site run by the Free Software Foundation.
The version control system FAViR uses is Mercurial. You can use this system to sync up your local codebase with Savannah's repository.
If you'd like to make a contribution or bug fix but aren't familiar with Mercurial, don't worry about it — just mail a patch or source code file to one of the editors.