Are you sure the 91fx is lighter than saito100? According to Tower the 91FX has a total weight of 24,3oz and according to Horizon the saito100 weights 20,9 muffler included.
But the quiet mufflers on the twostrokes are heavy and they also rob some power. If you install a quiet tuned pipe on your 91fx I am pretty sure it will become stronger than most 120-sized fourstrokes.