Steerer tube is deformed. Once the quill is out out, take an old seat post, put a bolt in the end and spray contact glue on 80 grit sand paper so it’s a tight fit. Use a lot of oil, put the bolt in a drill and ream out the fork tube. Replace the worn out paper and then use layers of sandpaper as necessary. Usually you can get the quill back in so it’s adjustable. If it’s too deformed for this to work it will be apparent right away. If it’s not too bad this can work and doesn't remove enough metal to be a problem. A reamer won’t work as it’s not aggressive enough and you will wear it out before the flat part of the tube is rounded enough. The quill might be tight getting past the deformed area, but that is what you want.
