Did you already replace stringers, they look to be in good shape.
As long as there in good shape and attached to transom ya should be good with 3 or 4.
My CV16 only has one down the center.
1972 and older (In USA) will not have the serial number embossed into stern.
They would have a plate on dash, back by transom/splash well or on control board where throttle control bolts to.
Under stern / splash well area, there might be a small piece of paper with serial number glassed in, on bottom were red circle is.