That's a bummer, Wayne. Since it's rusted through in that one spot, it's likely close to rusting through elsewhere. If it was only the bottom of the boiler, you could possibly cut a brass or copper disk, sized appropriately to fit in the bottom depression, and silver solder around the periphery where the boiler meets the firebox. Even then, I would do a hydraulic pressure test at 2x working pressure just to be safe.
I don't know of an epoxy that would withstand the heat from the burner, although the water in the boiler would absorb most of the heat. Still... maybe a good one to back away from. Just my thoughts.
Paula