Window Operator

Our window operators don't seem to get the windows all the way closed anymore, and there is no separate lock to snug it in tighter. Do you have a recommended replacment operator, or perhaps a recommended lock to add in addition? It's on the second floor, and we normally take the screens off in the winter, which is when we'd need to lock it.

My husband is very mechanically adept, so whatever you propose I know he can install.