The standard screw stud has a 3/8" screw. The stud manufacturers produce a screw stud with a 5/8" screw. This is used specifically when the stud hole is enlarged. If these do not solve the problem, you have to fill and re-drill a new hole. The size of the snap, on the stud, remains the same. Therefore, you do not need to replace the canvas snap.
You can replace the zipper pull with a new one rather easily. However, you need to know the size. Google Sailrite and Rochford supply. You can purchase all your supplies from them.
Bob