There is no perfect alternative. That’s because each type of starch is different and has its own distinctive proportions of starch molecule chains called amylose and amylopectin which result in different qualities, such as different gelation temperature ranges, consistency, or stability on prolonged heat. Potato starch, for example, has a large number of attached phosphate groups, causing the starch chains to repel each other. This means that it doesn’t congeal very well upon cooling, which is something we rely on heavily while making Turkish delight. All in all, from looking at the below chart from, you guessed it, Harold McGee's "On Food and Cooking", I would pick arrowroot starch as the best alternative if required.