Here's two alternatives on ebay from the usual dealers there
bicycle Motorized BIKE ENGINE extension muffler CHROME - eBay (item 150389295387 end time Dec-15-09 12:31:51 PST)
CHOPPER MUFFLER Motorized OCC Bicycle Motor Bike Parts:eBay Motors (item 310183160703 end time Dec-19-09 11:47:54 PST)
I bought the first one from boygofast- it'sa longer pipe and a longer muffler too. It is designed to run down and over the top of the bottom bracket- so the clearance is very limited at the cranks and even off the motor, past the down tube- it didn't fit on my Micargi cruiser with a 66 motor- but it does look like it will fit the Schwinn Traveler with 49 slant I'm building now-- so I'm happy, it looks great! but don't recommend for cruiser. The more seasoned mechanics can probably make it work.
The more pricey from livefast motors drops further down BELOW the bottom bracket, and so clearance is probably not as critical- and it's longer and probably more quiet- can't say anything for sure- haven't seen it and it's sorta expensive.
in the picture below the first pipe is just hanging unattached but will draw tight up next to the chain stay when installed.