Andy,
You would not want to drive on the road, but I would probably try clamping a 3/4" plywood "wheel" to my smaller steering wheel, maybe using some padded u-bolts on the rim or spokes, to experiment with different diameters of wheel to see how much improvement you can expect. Lots cheaper than buying a new wheel, and you can start big, try it and then trim it down an inch to see what difference it makes. Just a thought...