take it to a muffler shop.
they can cut the rusted peice off and put on a new one and weld it up
or adapt a new piece they have these new clamps about 4 inches wide that can clamp it on so that it is removeable.
As someone who has modified the OEM bends into a slip fit for cobra pipes I can tell you this.
It's not a 90. It's a stepped 90 and also it would be easier to just buy a used pipe and slip it on. Imo if you have $500 you can toss at the problem do as I did. Swap to cobra pipes with the 5/8 baffle if you can weld. Best cheap upgrade to date. Also decat the box like I did. Again if you can weld and have a tig and a purge manifold