Hi Mark,
This sort of weld up repair has been done successfully by a number of people over the years. I”m currently looking for someone to do mine after a previous welder I used has retired and no longer does any work. I made a special jig so the whole muffler can can be separated from the downpipes and then welded back in exactly the correct orientation. What is usually needed is a replacement section, like a cone, to replace a section of the muffler as it will be rusted out a lot on the inside. If you just try to fill weld it you’ll just end up seeing the rust break out again within a few months. It can be done… it’s been an ongoing issue with me to get someone who can do this as a service to members. Not got there yet.