That crazy ST "dustbin" muffler has always been a sore spot on an otherwise excellent engine. I like to search the swapmeets for older teardrop shaped ST mufflers that slide onto the same exhaust stack as the late model muffler. They aren't too hard to find and can usually be had for less than $10. Best I can tell there were only two sizes - .40 size (.40 - .51) and .61 size (.61 - .91).