Hi Chris..With a little modification you can use the swinging arm serrated washers. If memory serves, the hole in them is 5/16" (as opposed to 3/8" on the M20) so they will need to be drilled out. You will also have to get them faced off on the non serrated side to reduce the thickness to suit the available space and M20 stud lengths...S.Arm models have one washer as part of the bolt and the other one loose so you need to ask for 2 of the loose ones.
However, on the up side you should get a pair easily enough from one of the Goldie people or perhaps C&D Autos, Kidderminster motorcycles, Draganfly etc etc..Or you could look on ebay....Ian...