A Carl Gustav would be a 94 or 96 Mauser, made whenever they were before WW2. The action is basically the same, even the later Husqavarna made during the war, it's a 94/96 action.
The cracked bolt sleeve may be the issue, without seeing it up close its hard to say. Maybe someone futzed with the sear engagement and it's shallow and with the bad bolt sleeve is acting in combo to not reset the sear properly.
You may be able to get the firing pin assy with bolt shroud and everything attached. Then insert the complete unit in and have a few spare parts when done.
Whatever happens let us know what you did and how it works out.