Choose which pixels affect content-aware fill? Temporarily crop footage?

I'm honestly not even sure if this is the right question to ask, as I'm pretty new to After Effects, but know this is something you can do in Photoshop. I'm working with some uncropped super 8 footage and would like to remove this piece of dust that was at the bottom of the frame in my camera while recording. I'm having trouble as the content-aware fill tool is being influenced by the black border surrounding the frame. Is there any way to temporarily crop the footage or choose which are of the frame the content-aware fill tool references?

Attatched is a screenshot of a frame from the clip showing the dust I'm trying to remove.

Thanks in advance.