Well this made me go check mine its definitely not oem then, no balance or link pipes between the headers and no nut flanges, goes 4-2-1 and once i cut through the grime its stainless, need to scrub at the welds to check them as its thick in decades of crud.
so far looks solid
looking at the pictures if no welder available, try braze or solder, can use a map gas torch.
for a small areas like that other idea might be gun gum or the like and put a plate across the whole area, fill it with gun gum and either wire it up or use belly bands like very long hose clips to pull it all together.
