You could place a "wanted to buy" post on that forum here for tires. There are a few CABE members that advertise tricycle size tires for sale from time to time. There's always ebay, too, although some tire sellers are getting a bit ridiculous on prices lately.
The seat that has been installed on your Colson is actually a streamlined Mercury (Murray) tricycle seat adapted to fit this larger trike. A juvenile size (approx. 8" x 9") Troxel seat with sewn on vinyl or leather top cover would have normally been original equipment. A Troxel black rubber topped seat could also be used and be appropriate for the age of the trike, although Colson usually used an upgraded sewn seat on these larger chain drive models. This would look great all fixed up and was well worth grabbing at that sale!