While modern tankers are strong, they are not designed to take a torpedo shot. After reinforcing the deck to take the shock of repeated controlled crashes, all the additional armor to beef up the hull, all the self protection armament, all the ordinance for the ac, the addition of a hangar deck and elevators to move the aircraft to and from, adding an angled deck for recovery operations, moving the bridge, and everything else which would have to be done. It seems to me it would be more cost effective to build a carrier from the ground up. Tankers were not designed for speed either, they are steady. One of the reasons for the speed requirement of US carriers is to put enough air over the deck to launch aircraft regardless of the wind conditions.