Here is some other info I found on the KF mufflers....
As far as I know, the Y, elbow, and muffler were all Rotax
parts modified by Skystar. The Y appears to have been shortened, the
elbow shortened and tightened, and the muffler was a 180 degree unit
that was cut and welded to turn double 90 at right angles plus the
outlet was replaced with an elbow and extension to clear the bottom.
Ask John McBean at KitfoxLLC, but I'll bet you'll have to
have one fabricated. Dave's already given you some guidelines, but
watch the 660mm from exhaust flange to muffler box. (Look at the 582
installation manual on line.)
Guy Buchanan
San Diego, CA
.(2)Well the Y pipe is what was cut I think at Skystar as well as the 90 Elbow but I added mine to the elbow. Your elbow will be near the cowl and might burn a hole in it but a air outlet could be added there or just a bump. The muffler will not longer be straight below the engine but on a little angle. I am sure there is a way to alter more but I have not spent the time on it yet.
Basically I just added 1 1/2 inches to the header directly onto the elbow on the top of it. I could have added to the y pipe but it seemed easier to do it this way.
Just out for a 1 hour flight with a friend , I have on the IVO medium right now and it worked well about 4 mph faster that without the exhaust mod.
Now all we need to do it put a different pipe on it and see another 5 or 10 hp