I have replaced a number of jockey wheels on Cobra trailers with any number of solid rubber wheels from different manufacturers. None of them were actually "plug n' play." They all required special spacers or axles and other hardware. That being said, it isn't actually rocket science. It just takes some time to find or fabricate the necessary parts. I have had some luck with this one:
http://www.harborfreight.com/8-inch-...ire-42427.html The 8 inch might be too big, but there are others available. Note: I don't recommend the solid plastic wheels found on many front trailer jacks. They break too easily.