One piston engine, silent exhaust

I have a gasoline engine, model 168 (6,5hp). I need to modify exhaust system. Current one is to loud. I am looking for option to be as silent as possible. Muffler size can be max round tube, 77mm in diameter, max 20cm long, tube from engine to muffler is 1 inch in diameter. No need for power,... just not to be this loud. See picture:
[IMG]http://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=1988&stc=1[/IMG]
Any idea?
What to use in a muffler to lower the sound? Tubes, walls wit holes, foam,...?
I am looking in something like this:

[IMG]http://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=1989&stc=1[/IMG]


or

[IMG]http://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=1990&stc=1[/IMG]
or
[URL="http://i52.tinypic.com/zx5r9w.jpg"]
[IMG]http://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=1991&stc=1[/IMG]
[/URL]or how to design?
How it's done in motorcycles? If they lower the sound... ;)

Designing and building a muffler seems like a lot effort. I suggest that you look around for a similar engine and determine if an existing muffler can be procured and installed.

Check out those recreational run silent generators, like the Honda EU's or yamaha's.