Why pre-dreadnought carrier conversions were unattractive

Which old ships would navies convert into carriers if treaty limits allowed it?

Armoured cruisers are the likelier candidates; most pre-dreadnought conversions would demand such extensive reconstruction that they would effectively be new ships.

Conversion depends on hull length, speed, machinery, and material condition; the availability of an obsolete ship alone is insufficient. Armoured cruisers are the more plausible candidates because their speed and proportions better suit flight operations. A pre-dreadnought would require such extensive work that the conversion would approach a new ship built around the surviving keel. The comparison is with the Royal Navy's deep wooden-ship refits, in which almost every component could be removed and replaced. A serious rebuild would probably add turbines, small-tube boilers, and a heavily altered hull as well as the flight deck. The nominal saving of an old hull therefore has to be weighed against the cost and complexity of reconstructing nearly everything that made it an old battleship.