You don't need to 'buy the rights'. Go to the same place I went to get the info. Go to GitHub and just fork it. Then, maintain your new forked version after rebranding it SpongeBrowser. It's opensource. It's free as in beer and free as in libre.
That's all I did. I went to GitHub and checked to see when they last made a commit to the project. Go there. Log in. (Create an account, if you don't already have one.) In the semi-upper-right, click on the button that says 'Fork'.
If you don't want to fork it on GitHub, you can clone the repository or just download the source in a .zip format.
Either way, nobody needs to buy the rights. The rights are perfectly free. Welcome to the land of opensource. It's pretty awesome here.
Here, I am being less lazy. It's licensed with the MIT license. I checked. Fork it all you want. https://github.com/beakerbrowser/beaker/