I see no reason why you shouldn't be able to replace the bearing. Just because it is sold as part of an "assembly" that Yamaha sells as a whole doesn't mean you can't get one. After all, a bearing is a bearing. Yamaha (or Motori Minarelli) won't make the bearing, they'll buy them in from a bearing manufacturer, and you can do just the same.

Has it got a bearing number on it? If it has, then use that to source a new one. If not, then take it to a bearing supplier who should be able to match one up for you.

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