You may be able to fill he hole (drill, tap, plug screw, and file). But, it's probably easier to buy a replacement.
Here's a link to the part listing. Sometimes the parts that are listed with Numrich are mis-titled, so it can be a bit of a crapshoot. Also, I'm unsure if the bolt for the Crass model is longer than that on a Lewis. Of course, you could cut and file the longer one to the proper length for your shotgun. Anyway, try this one:
http://www.gunpartscorp.com/Products/1007820.htm