Ive seen some very nice mufflers made from brass or copper and brazed together. They have a tendency to get a little heavy if your not careful but it has been done.

For what its worth Ive done a bit of custom work; aluminum welding for other modelers and Im always willing to if someone has something they need made up. I work pretty cheap and Ive done work for modeling supplies.
There are just some combinations that nothing is available for and they are to odd for some manufacture to tool up to make one or two.

If anyone runs into a dead in like that dont hesitate to contact me and we may be able to get something done.

Adam