Hmm, I'm not 100% sure as I've never see it from the side of someone buying the game, but I believe if you have bought the game, are logged into itch and are on the game's page, there should be a button mentioning request key on the purchase page.
There are definitely keys left, so if the button is there it should not be a problem. Please let me know if the button is not there and I'll do some more investigating.
Thanks for playing