I don't know what the outside diameter of the exhaust pipe is on the C5.
If it is the usual 1.75", then any typical 1.75" inlet-size Bullet muffler or Harley muffler will work.
You can also get copies of the old Brit mufflers in that size too, and if it is smaller, there are some size variations made to fit different size pipes too.
It's pretty wide-open in terms of selection.
You may have to make up your own mounting strut, if the bracket isn't a perfect match.
I think 1Blackwolf1 has a Dunstall copy that he's not using, and you might contact him about that.