A friend of mine has a gravity furnace built in 1927. It sprung a major leak and was rendered useless.
The system has a big 3 piece cast iron kettle with 4 tie rods holding the castings together, over a forced gas fire pit. The pit is brick lined and the kettle has a brick house around it. Of coarse, the furnace and the pipes are all coated with asbestos and there are sheets of asbestos on the ceiling, DO NOT DISTURB THE ASBESTOS, WEAR ADEQUATE PROTECTION.
We broke out a few bricks and found the leak in the top casting.We hired a welder to braze the hole and the top casting cracked, this created more problems.
My advice is to use jb weld in a heavy application over the leaky area, DO NOT WELD THE THING! you must drain the system of water and clean the casting well before applying jb weld, but the stuff works great and is rated for high temperature.