I'm assuming he has a 2-stroke, not 4-stroke. I'm sure J-tec, slimline or bisson will make a muffler that will work. The cowl is very narrow and you may have to do some carving out to make it work. It would bolt on the side of the motor with two tubes pointed down out of the side of the cowl. The TF in-cowl won't work with a .91 size. Good luck.