Hi Bob, thanks once again fro your detailed reply.
Given the tyre sizes you have stated, the wheels on mine seem to fit, it's just the current tyres that are wrong and I can replace those with the size you have detailed. This will make a difference in ride height on the rear only of about !/2" but the width will reduce by about 2 1/2" and the front will be about the same diameter but will reduce in width by about 1 1/2". That should make it look a lot better and give it more tyre clearance at the rear.
the mower certainly seems to share parts with the mini tractors and as such the anti scalping wheels should, as you say, be 5" diameter nylon ones but having said that, the mini tractors had smaller wheels, 4" front and 6" rear.
I will be pulling the engine mounted clutch apart this weekend so will be taking copious photos and measurements so watch out for these early next week.