Looks like total replacement's your best alternative, Joe.
Did you do a lot of big cooks on it? I had a Char-Broil Silver Smoker offset for a couple of years and while I liked it I never took advantage of its capacity, so when I had a chance to pick up a used WSM I went that route.
The fuse method looks like an interesting alternative for a horizontal smoker. If that's the style you like what about getting a really nice one (heavier metal) and adding the firebox at a later date if you decide you need one?
I don't do as many big cooks as I have in the past. There are times when a brisket or two pork shoulders may share the cooker with 6 racks of ribs. I have learned to juggle of course, by adding the ribs (st louis cut) to the cooker after I have foiled the brisket or pork. I have placed the foiled meats on my gas grill, with one or two burners off to complete the cooking process.
I like the side fire box concept. I posted a thread on this site (Chargriller?) asking if anyone using the Chargriller Pro, which is on sale at Lowes for $119 (without SFB), has made any mods to take advantage of the large unused space under the grates. I haven't received any replies yet. Seems like alot of wasted space.
I would appreciate if anyone can recommend a moderately priced SFB equipped smoker, other than the usual Lowes or HD stock units.