Ever since I put the UNIMOG together I have wanted to replace the tires with something more scale looking. My choice is the Pro-line Interco Super Swamper in size 2.2 on a set of standard TAMIYA wheel that have been painted green. I still have to paint all the screws and the wheel hubs green.
Regarding the foam in the tires I found some star cut foam on ebay. It should give better traction. Then there is wheel weights: I haven't added any yet. If I can avoid wheel weights I would prefer that as I don't like adding weight.