I think you will have to make your own as all the actual engines had straight spokes with six spokes, you want 4 spokes for the actual plough.
You could either fabricate by making an outer rim from about 5/32nd round bar, cut the spokes from 4 bits of 1/16 steel and silversolder them to a centarl hub. Or turn from solid forming a rounded edge to a disk that is thinned to 1/16″ in the middle and then cut out between the spokes, thats how I did the brake handwheel on my Fowler as the casting is N/A.