Using an existing project usually gives you a major advantage and goes against the spirit of most game jams, which are meant to challenge you to create something from scratch under the same time limit and constraints as everyone else.
That said, this jam’s rules specifically allow pre‑existing code and licensed assets, as long as you credit anything you didn’t make yourself. So it’s not against the rules here — it’s just something to keep in mind if you want the full “from scratch” jam experience.
The general guideline is: don’t submit an already‑built game or large codebase, but small reusable pieces or assets are fine if the jam allows them. Starting fresh usually gives you the most fun and creative challenge, and if you want to grow as a developer, it’s often the best choice. Let the theme shape your idea, and use small assets to help bring your game to life.
Hope this helps!