The MyBB 2.0 source code has been released in the official github repository.
It’s still at an early stage, the MyBB core team does not call it alpha yet. You should not use this version in production as it does contain bugs and breaking changes will probably be made until the MyBB 2.0 release candidate is published.
If you want to dive into the source code of the new, laravel based, MyBB 2.0, you can do it here in the official repository. Be aware that no support is offered for 2.0 yet and only experienced developers and plugin coders should work with this version.
To install the current state of MyBB 2.0 you’ll first need a Homestead environment with mybb2.dev pointing to /home/vagrant/Code/mybb2/public.
If you have setup the environment you can clone the git repository and install the dependencies using composer.
The full installation process is described in detail here.