You might be able to bore out the omni wheels. Bore a hole that is the same size as the motor shaft or the same size as the hub. Getting a good press fit is hard but you can use an epoxy like JB Weld to make a loose fit very solid.
The last couple projects I did I made aluminum bushings. Get some (say) 1/2 inch diameter aluminum rod. Bore a 1/2 inch hole in the wheel and glue a section of the rod to the wheel hole. Then drill the rod for the motor shaft. It helps if you have access to a lathe but you can do without if you are careful to make jigs and clamps to guide your tools. Even a $59 Harbor Freight drill press can make this work