Looking at this, it seems the spark arrester (#7 on this) is $63. The lower part is just the spark arrester screen, which slips off (not sure which post showed this). So, I think Ed@Ford posted he modified the muffler on his XR400. It seems a cheap mod would be to get the SA out, and enlarge the holes (which you can't see in this schematic) to make it a bit more free flowing, and a little louder, way less $ than any aftermarket muffler, while still retaining the cat. Point being if you went a little at a time, enlarging the holes, and got too far, easy enough to put it back the way it was. Some of these DB ratings I'm reading about are just way to obnoxious, just my 2¢. Will the ECU "relearn" the mod?