I wouldn't be too picky about how to fix it up "properly." That bike might have never had a tank or a rear rack. It might not even be a Spaceliner. That frame was built by Murray and assembled into bikes for a number of brands, including Sears and Western Flyer (see photo below of a Western Flyer version.) Some were sold under the Murray brand. In any case, If you want a rack, find something that looks about right, paint to match (or contrast), and bolt it on. A tank that fits would be more difficult to find and it may be that your frame doesn't have tabs for securing a tank. Check with your mother-in-law; she may have some ideas about what would look cool. Be creative and have fun!
View attachment 1228801