The proper way to repair an exhaust manifold leak is to use a piece of sheet metal and weld it onto the damaged area. A cracked manifold will continue to grow in size if not repaired properly and promptly.
Before the sheet metal can be used to patch up a hole, the manifold will need to be inspected to find out what type of material it is made from. Cast iron manifolds use a specific type of technique that requires the manifold to heat up enough so it can take to the welding. Following proper technique for preparing the manifolds surface and using the correct solder will allow the welder to create a strong patch.