Yep! Go for it! This challenge is not about finding an idea or proving that you made it all by yourself during the duration of it, it's to refine your scoping, planning and execution skills while preventing the crunch that often happen if those things are badly managed. So yeah prepare yourself, think about what you'll work on, when and how, it's totally okay. I even think that for this one it's better to start with a clear idea/concept of what you'll work on or even a started project than to start with nothing.