Central Boilers have "stay bolts" welded between the inside of the water jacket and the outside of the fire box for support. It is somewhat common for older CB's to start leaking at these welds. They are located near the middle of the fire box walls, it appears that is what has happened to yours. I had the same thing happen to my 1996 model last year. I actually had leaks develop on both side of the firebox at the same time last year and they were both at the stay bolt locations. I am not sure that I would advise cutting out the area because most likely its not a case of an area being rotten but instead its a leaking weld. You cant see the welds because they are on the outside of the wall. I fixed mine by welding a 4"x4" patch over each area.
I would recommend getting a welder that has experience welding on boilers. I had my brother in law weld mine, and although he has been a welder his entire life and is considered pretty good at it, he struggled getting his welds to be water tight.