My local hardware store has an assortment of steel and aluminum spacers that you can buy. They come in different widths and diameters so you might find something that will work and look good too without going thru the chore of making them or stacking washers.
From the pic you posted it would appear that the stock split collars and stock flange were placed outside the welded flange on the pipe to take up the space. Shorter studs, a stack of washers or a tube of correct ID, OD and length would be the quickest fix if you do not have the stock split collars and stock flange.
The OP said the flanges are not welded, but are captured, so split collars should work just fine.
Maybe I am confusing myself with terminology. The flange I am referring to Kawasaki calls the exhaust pipe holder. It fits over the threaded studs and transfers the pressure from the nut to the pipe, via a sleeve welded onto the pipe.. However, in re-looking at the picture, if the pipe holder was as far out as the wear marks, the studs would be too short to reach it. Some pretty short split collars would be needed, or some spacers as discussed above.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough