You could get everything you need from an ST, but you would need to get a complete wiring harness, new controls with the switchs, luggage racks with turn signal mounts and some other parts that i can`t think of right now. This would be expensieve and a lot of work. I don`t know of a cheap easy way to do turn signals. If you could get lucky and find a good set of old cat eye signals, they would be the easiest. But finding a complete set that works is hard to do.
Wish i had a better answer.