your setup for magic would be: staff of the dead OR staff of light, casting fire wave tome of fire occult necklace slayer helm (i) less important: ahrim's robetop + robeskirt any kind of magic boots (eternal best, doesn't matter too much though) seers ring (preferably (i)) tormente...
Something that some player do is build mage items on Corki and although some of those items may work in some situations, like Void Staff if the enemy is building more items that have magic resist and yeah that is a good build but keep in mind Corki scales more with AD than AP, so yo...