Mark,
Press the shaft out of the motor and make a complete new shaft with integral ER20 collet chuck. Plus a new drive end housing to take a better bearing and a bit more substantial.

Hopefully motor will be here tomorrow and I can get it stripped and measured up. I can only work on this two nights a week, Tuesday and Friday as I'm doing it with my grandson as a design project, unfortunately I won't be able to order the bearing until the motor gets here as I don't know which one is fitted, I'm guessing a 20mm bore but it may be a 25mm bore.

Mind you nothing to stop going up to 25 which I may well do. Watch this space for a Friday update.