I've got a catalog from Agri Supply that shows a lot of different races and bearings for disks. It does not list the disk to be used on so maybe these are somewhat standard. For instance, they sell the spools for spacing, bearing covers, bearings themselves. Maybe they have the cover you need. They have a web site - www.agri-supply.com. I've purchased hydraulic cylinders, over running clutches, trailer fenders, etc from them and been happy with their products. Go to web site and search for bearing spool or just bearing. They sell the disks in notched and un-notched in diameter from 12 to 36. The 12 are $4.
If you have the old part, you may be able to weld it. Or perhaps a new one could be made since you probably have good ones on the disk for a model.