One is inclined to wonder about why it should take so long. Obviously it's something of an apples and oranges comparison, but the conversion of the Akagi from her weird triple-decked design to the conventional full-length flight-deck system took from April '37 to August '38. Admittedly they were simpler ships, but still, I would guess (very much a guess, of course) that the requirements of an aircraft carrier haven't changed too very much in terms of what you have to have in it - i.e. space to store aircraft, fuel, ammo, stores etc. You have to wonder what could be done, if the Russians pulled their fingers out. having said that, Akagi was, for her time, pretty big. Big enough, fortunately, to accommodate the ever-growing sizes and take-off runs of her air wing. But still, I wonder.
Mind you, I know slightly less than bugger-all about the intricacies of aircraft carrier design, so I am now hunkering down in preparation for a bombardment of corrections.