make up some hubs and use a 5/8" rod for an axle.
Than you can buy plastic or nylon rollers at any boat store that work great as bearings.
So cut the plate for both sides of the rim, bolt them together, drill a hole in the middle for the tractor bushings, then use cotter pins to hold the tires in place?just about anything will do for the hub. It's not like it's going to see high speed or great loads.
1/4" aluminum plate should be more than enough and easy to cut. Drill the center hole to 7/8" and use a lawn tractor plastic bushing as a slide.