Is This Exhaust Manifold Repairable?

It appears to have been brazed so that would all have to be ground out. You can fix cast iron with brass and it will hold up well.
The standard fix would be to V out the crack and fill with a nickel rod (they make nickle wire for mig also). Pre-heating the part is a must and a slow cool down is important or it will crack along side of the new weld. We used to cover the part in sand for the cool down process.
Don't thorw that manifold away. You would have no problem selling it if you decide not to fix it.