Anything you can get it to eat. Be aware that it usually doesnt go well with cross beak as it tends to worsen quickly. Not many I have heard of survive for long
A mild cross may work but the more severe the less chance of survival.
I have a top hat chick with a cross beak a bit worse then yours - my vet is looking in to clipping the beak once she gets a bit bigger - she is doing some research on it. Good luck, mine likes oatmeal, soaked egg layer crumple and bread crumbs