When the body has cracked, as in this case, the usual repair is to glue it (with wood glue) and clamp it while the glue dries. That usually solves the problem quite well. Just be sure that the entire crack has glue in it. When doing this sort of repair, I usually make a flat blade out of very thin metal (from a soda can or sardine can lid) to shove the glue up into the crack. Make sure the box aligns well with the bellows frame while the glue is drying under the clamp ... otherwise, you might not be able to fit the bellows to the accordion afterward. For this reason, you should probably have the bellows assembled to the accordion while the glue is drying. In that case, be sure none of the glue can seep to the bellows frame, which would permanently attach the bellows to the accordion. Putting some waxed paper between the bellows and the accordion at that location while the glue is drying can avoid this problem. If you do that, the screws should be unnecessary.