First double check that the problem is in the headset/steerer area as follows.

Apply the front brake to lock the front wheel, then hold the lower bearing area and even the crown while rocking the bike back and forth feeling for any movement. If you feel movement, then it's either a headset adjustment, or possibly a cup is loose in the frame or the crown race is loose on the fork. Sometimes integrated headset bearings don't seat perfectly and are able to shift a bit when braking.

If you can't feel anything, it's possible that the problem is in the brake itself or the rim joint is imperfect causing the knocking.

Lastly, I don't know how bad it is, but if not too sloppy it's OK to ride it as is for a while. It's less than perfect, so no 50mph downhills through mountain passes, but otherwise don't sweat. After all you've been riding it that for a while already.