I have 2 KwikFlys myself, one with trike setup and the other with a "kitbash" taildragger conversion. Trike weighs in at 13lbs (!!!) and is powered by an unpiped ASP61 with stock muffler (it had a HB61 from the late 70s until it went sour with a lack of compression). It flies very well!
The other weighs in at about 10lbs, has a SuperTigre G75 also with stock muffler, and has amazing vertical (not quite unlimited though).
My opinion: leave those ugly ugly pipes off the plane!!!! They look soooooooo awful. You will be fine using a stock muffler and the 91, or anything down to a 61 if you keep the weight down.....
PS: retracts on a KwikFly? I've thought about it....decided not to, though :-)

Regards
Hank