Repairing a cracked, damaged or worn wheel
The wheels wear out and are no longer usable after 1-2mm are worn off the sides.
Light wear and cracks
A quick fix might be to spray paint a worn wheel with a polyurethane coating. I have no idea if this would work: http://www.utilitysafeguard.com/K07120-Clear-Polyurethane-Coatings-Matte
I could be wrong, but paint is general 0.1 to 0.2 mm thick. This would require many coats to achieve 1mm.
Heavy wear and notches
It is possile to use M-1 sealant from bestmaterials.com to fill gaps, cracks, and flat spots.
Retreads.145" X 12" X 12" URETHANE/POLYURETHANE SHEET 75D and cutting it into a disk, approximate 4.5" in diameter and 1/2" wide to match the sidewalls of the tire. Then polish down the wheel the the appropriate width and glue on the disk. It would not be necessary to bend the polyurethane sheet because the sidewalls are flat.
Poly Tech Innovations sells an adhesive for bonding the two urethane together, 1/2 pint kit is 20.95.
Poly Tech Innovations
35283 340th Street
Lake City, MN 55041
As of January 2009, community members have been unsuccessful at ordering replacement wheels from Roth or its dealers.
The closest match to the required measurements that I have found is the WEZ-0515-MOPP by Ez Roller. It is too narrow by about 5mm, but potentially this can be made up by placing polyurethane tubing over the motor drive shaft cone.