I know this is an old thread, but I just had my exhaust done and I though my experience might be helpful to someone.
The $WS exhaust is no longer available. I want my car to appear factory stock but not necessarily be factory stock. We picked a muffler from a 92 MX3 GS; it close but not exact and the mounting points need to be moved to match the hanger on the car. The biggest difference is the length of the exhaust tips exiting the muffle. On MX3 tips are shorter and straighter. It works and you would really need to know the 4WS cars to notice the difference. On the other end of the muffler, a short "splice" had to be made to connect the MX3 muffler to the factory intermediate pipe.
The second time I needed exhaust work, the intermediate pipe went, so a new pipe was made to connect the CAT to the muffler. Again, if you really know your 4WS stuff, you'd notice the flange would be in the wrong place.