An alternative for the 91FX is the Jett Turbo in cowl muffler, which should not only fit but increase power.
http://www.jettengineering.com
We have a number of the 91FX's and have found that they leak at the center ring which we fixed with high temp silicone. We've also replaced broken muffler bolts with threaded rod using double nuts on each end.
Good luck!