In addition with other comments, you Can look at what are your contributions and code changes. All the parts that are completely reworked by you could be consid

In addition with other comments, you Can look at what are your contributions and code changes. All the parts that are completely reworked by you could be considered as yours. And you can also ask them to ensure that they don't use your own code/modification as a retaliation of the code not being open source.