I'm probably going to answer my own question because I ordered a set of ball bearings to repair the extruder and then walked passed my lathe and realised all I had to do was turn some brass bushes the same 10 mm OD and 6 mm ID to replace the ball bearings. Thirty minutes later the printer was back online.
Now thinking about it, I wonder why ball bearings are used. There is no critical clearance between the filament and the drive cog as the filament is spring-loaded against the drive. Thus as the bearing wears a little over time it's only a matter of increasing the tension. There would be no sudden collapse in the middle of a print job. Of course, there might be good reason to once every 100 operational hours to re-grease the bearings. There would be plenty of warning of impending drive failure and plenty of time to figure in a new bearing. I expect however the brass bush to last the lifetime of the printer.
Has anyone been down this path with any sort of success?