After contacting and speaking to a couple of motorcycle exhaust parts suppliers, I discovered that these rusted out, broken flanges are hard to come by. No body knew were I could get hold of some kind of weld in repair kit or anything. So the conclusion was that I would have to make something up myself.
To my horror, on close inspection, it turns out the previous owner had already discovered the same problem and had done a bodge repair - trying to fill the cracks with bird poo style MIG welding, then ground it back. This leaves poor penetration and a messy repair. I decided to cut the rusted out flange clean off and try to weld on a ring. Hopefully, with decent penetration it should be an ok repair that will last.
I got out the budget miter saw jig to cut a straight 5mm ring. Then trimmed down the other flange to compensate for the added depth. I then cleaned up both work pieces ready for welding. When I did this, a piece of the crappy weld felt out which meant I had to fill this with weld when adding the ring.
Once the ring was seam welded along its edge, I ground down the exhaust gasket contact face to make a good seating surface. It’s not the prettiest repair but it turned out pretty straight, wide enough and hopefully strong enough. Now just gotta do the same thing to the other busted one!