Called my mechanic this morning, and he referred me to a local muffler/exhaust place. Took it there, and he said that he could easily fabricate a new exhaust pipe. He has 1 1/2 inch pipe and a pipe bender. :-) I'll take it back on Wednesday morning for the fix.
I did also call the nearest Cummins/Onan service center, and they said that it was very difficult to get parts for a unit this old, and they weren't wild about replacing the muffler. I would need to drive it there (1 1/2 hours away) for them to look at to determine if they could even replace the muffler. It looks to me like you would have to drop the whole generator to replace the muffler ... don't want to get into that unless it's absolutely necessary.
Thanks for all the advice--this forum is an excellent resource!