The effect, apart from failing an MoT test, is that the bike feels odd when going into corners as if it doesn’t want to lean over, then it seems to tip suddenly and also seems to increasingly ‘tramline’ on rutted surfaces.
The cure is to lower the front wheel and yoke assembly out of the headstock, and drift out the lower race from the frame and replace it. You can do the whole lower bearing if you want or if you use the same bearing manufacturer as previously, you can just do the outer race. Saves time and effort.
apply lots of grease to the new bearing as this not only lubricates but also protects from corrosion and dirt. Bearing sizes are the same on all X9’s and the sizes are on the parts list pages.
You may need to fabricate a long drift bar to reach down the headstock to drift the race out as there isn’t a lot of room, and also re-fit the lower race the correct way up!!