If Koya leaves the battlefield before the delayed triggered ability goes on the stack at the beginning of the next end step, the delayed triggered ability will still resolve as normal. You'll still get the chance to pay {3}{B}, and if you don't, the exiled card will still return under its owner's control.

If a token is exiled this way, it will cease to exist and won't return to the battlefield.

Auras attached to the exiled creature will be put into their owners' graveyards. Any Equipment will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist. If the card returns to the battlefield, it will be a new object with no connection to the card that was exiled.