Although I am not the creator of the deck, I think that this card is key. I think that if you manage to be able to craft the rest of the deck I would go the extra yard at craft this. If you still are hesitant, holy nova is the best replacement but still no where near as good as psychic scream is.

Psychic Scream is a great card. This deck plays more like an OTK deck more than a control deck so all you need to do is stall until you have your full combo. Psychic Scream is great stall completely wiping the entire board, this includes deathrattles, so why would it be bad? You will almost never have more control on board than your proponent in the later game and if you do, you have already won. Basically, this is like lightbomb but significantly better.