When the camshaft is still on the head like how the pic dipicts, and laying it flat with valves on the bottom will bend the valves. A slight bent is all it takes for new valves. That missing piece is from detonation. Got very hot in there and instead of the spark plug igniting the a/f mixture, heat ignited the end gas while the spark plug ignited the other end and when to flame fronts collides, BOOM. The block is in need of a rebuild. The head may be ok if you're lucky the valves aren't bent. Here a vacuum check is $30-40.