It should send out lateral branches which will both flower but with slightly smaller heads. And the whole plant will probably be somewhat shorter than it might have been, but as it's a mammoth, that still should be fairly tall. I have in recent years been purposely pruning some of my taller plants to achieve a greater fullness, and somewhat lessen the need for staking. Not that a sunflower much needs staking. :) And I wouldn't bother trying to get the chewed off top to grow.

Thanks for the info!