[chappyman66]

Kev is correct, GB Track and Fly were basically the same company.
In any event, finding replacements is a bit of a challenge although they are still around depending on the car.

It sounds like you are dealing with front wheels, although the rear hubs also crack.   If they aren't completely broken you can glue a ring made from styrene tube around the hub to support and reinforce it.

If you have a 3D printer you can print front wheels.

Tires are available from several current manufacturers.  Fly and GB Track cars are notorious for tires cracking.