the exhaust muffler isn't as near important as the size and shape of the pipe running from the head to the muffler. but replacing the stock muffler alone will help. Here's the one you will want to use.
http://abc.eznettools.net/ez-catalog/X329438/7/375
stay away from 2 stoke pipes. they are set up different from 4 stroke pipes and can sometime hurt more than help. It has all to do with pulse timing, valve overlap, size, shape, and a lot of other techno jumble.