Buoyancy Control

Hello everyone I am new to this site. I am so happy to find a site made just for daysailers where hopefully many of my questions can be answered. We currently have a DSI from the sixties (not sure the exact year). We bought it cheap and fixed it up. however we have been having problems with the boat rolling. Out of the five times the boat has been taken out, it has capsized twice. When it goes over there is no time to get out to the mast to put something under it and it quickly turtles. I know these smaller boats are prone to flip from time to time however I did not think this often. We learned that there was foam under the benches of the boat, however the previous owner removed it all. We assume that this is the cause of out problems rather than operator error. Does anyone have any suggestions on how to add buoyancy or correct this problem? Heard of putting swim noodles under the benches but not sure if this will work. Thanks