Generally, repairing a cracked block is considered a bad idea. It has been done in some circumstances but the life has not been great. A good resource for info on this subject is the dodge truck forums
since some of the cummins 6bt blocks cracked in the years close to 2000. These were known as the "53" blocks after the casting number. Depending on the size of your crack, you may get some extra life out of repairing it or it may not be feasible. Regardless, it is unlikely to be a good long term reliable solution.
Welding a block can sometimes be done but is not a good solution. Welding cast iron is very difficult and requires special knowledge, it isn't like steel
. In addition, you end up with stresses in the material which make it likely to crack again.
Another method that is popular on the cracked B series engines is lock and stitch. Basically, you stitch up the side of the block with a bunch of threaded inserts that self lock. I have no experience with the method but it supposed works reasonably well.