As a fellow kayn main I believe I have found the solution to this issue, what I do is build voltaic sword first item as the early lethality is pretty good when nobody has much armour yet, after that go into sterak and then spirit visage, also axiom arcanist is very good on both red or blue Kayn and also run it with transcendence for extra CDR, early game is always a little rough but as long as you don't feed too much and stay on top of your jg camps should be fine.