Engine Block Repair
Most cast iron engine block repairs are done using metal stitching or pinning. Welding an engine block can be difficult due to high temperatures and special procedures implemented by professional welders.
Use pinning to repair both small and larger cracks in cast iron cylinder heads and blocks. These pinning kits use a bolt of the same material as the head or block being repair. This is typically iron. 
Engine pinning is done using a drill, tap, bolt, and a grinder. Some kits include a glue or epoxy to finish the bond. This method draws the two sides together creating an almost undetectable repair.
If the crack is easy to get to like on the front of a head it may only take a few minutes. Welding a block can exasperate the situation and is usually used for holes not cracks in the block anyways. This is an excepted form of repair but as mentioned earlier left for a professional welder.
