the shaft will probably be a 3/4 inch dia bar with a keyway in it. you can just buy rims and tyres to go on this bar, but otherwise im sure it wouldnt be too hard to make a hub, or just weld a hub on the bar. I dont know of any company that makes such a hub, but im sure it wouldt be too hard to make or be made at a machine shop.
First I would look into bigger wheels that fit on the existing axle.
Just remembered, saw a mower today (westwood) with a hub that sonverted the 3/4 bar to a 4 bolt hub, I remember thinking, cool idea! That just shows there are some around, I will check tomorrow if it has a part number and manufacturer on it if you want, think itll still be there tomorrow......
Hope this helps, dont suppose you have any pics of your tractor, just to make sure were talking about the same thing.