Bottom wheels

Hello,
Both of these wheels sit in the plastic fittings shown here and that sits in a receiving channel below. there doesn't appear to be any method of adjustment other than adding a plastic shim where the plastic fitting hits. The main issue appears to be that both these bearings are busted. Maybe all i need is new wheels, however, if there are more parts that go with this that might improve operation i would want that. thank you so much.
-Jamie