I would try soap and water and a brush. The rubber itself looks to be in rough shape so if you care enough to clean them, I would just replace them. You can even upgrade the wheels while you're at it.
Keep the gray plastic. Remove black clip around the vertical wheel post in the gray plastic. Remove wheel and wheel post. Buy new wheel. Installl new wheel. It will be easier to find a new wheel once you have the old wheel out so you can take measurements. but it's likely something pretty standard, off-the-shelf. Wheels are something that companies buy, they rarely build them themselves. They typically come as a castoring assembly with wheel, axle, spindle, and attachment post in a variety of common sizes and with a dizzying variety of actual wheels.