I was thinking that it could use a Mac's tuned pipe header. The engines aren't drilled and tapped for a bolt on exhaust so you would have to use a clamp on unit. But it would be easy to make a tuned pipe header into a clamp on one. Another thought is that the Dubro Exhaust extension would work. You could cut it in half, and drill and tap the sides for a muffler clamp and then you would have two exhaust stacks. http://www3.towerhobbies.com/cgi-bin...?&I=LXD621&P=7
On one of my old Speed .61 engines I had to machine the exhaust outlet flat as it was still rough and unmachined. At the time I felt that was pretty sloppy for Webra to send the engine out like that. I fitted a clamp on Davis Diesel Development .60 size Soundmaster muffler to it.
Before correcting the problem
After correcting the problem