You can run the muffler as is no problem. Its just annoying to hear that clanking all the time and everyone who sees you fly will come up and tell you something is loose on your plane!

If you take the muffler off from the stack you can actually fold up the baffle and take it out so that it wont rattle anymore. Just be careful not to scratch up the inside of the muffler at the neck or you will have a tough time getting it on and off the stack.

The pitts muffler is an option or tuned pipe or ultrathrust or jett muffler. I recently upgraded mine to a jett. I had a macs pitts on mine for a while and it ran well. Some folks have to crimp one of the exhaust stacks closed in order to get good backpressure.

Of all the options the pitts muffler is probably the cheapest and its easier to install. With my jett muffler upgrade I had to do some major mods to the fuse to accomodate the long muffler.