1944 wasn't 75 years ago, it was 68 years ago. How could your grandfather have hunted with it until he broke his back 75 years ago?
Mismatched serial numbers on the receiver and magazine box means it is a parts gun. It will never be all matching.
Numrich has K98k stocks for a out $200. Some of the other hardware parts are available as well. You could easily put $500 in parts into it and still have a $300 rifle. It's a shooter, just shoot it.