Private fork on github with updates from upstream?

Question

GitHub

Please tell me how to get such a model?
I need an up-to-date third-party project that is used as an engine - now I just clone c of its repository. I'm not going to make changes to the code yet.
This project has files (let's call them data) that I write and that are processed by this engine. This data should be stored in my private github project. In the engine code tree, I can place it in a separate directory.
How to have up to date code and work with my data in github?