I have the same generator manufactured in 1998 with 1175 hours on it. You can do this yourself, no rocket science, you'll be fine. With two eyes, two hands, and some basic mechanics tools, you will be done in 15 minutes or less. No need to pay $125.00/hour labor for this. Its important that you buy a new muffler WITH a U.S. Forest Service approved spark arrestor like the one you currently have. Unfortunately that means that Onan is your best (not only) choice, BUT, not the cheapest. (no affiliation)
Spray some PB Blaster on the treads about 24 hours in advance and then again just prior to removing the fasteners to minimize the chance breakage.
Chum lee