Cast iron fireplaces come with mantels 1) as an all in one casting with the fireplace, 2) a separate cast iron mantel bolted on, or, 3) cast iron fireplace with wood or stone mantel bolted on. You don't say which you have.
Replacing 3) should be fairly easy. Replacing 2) with wood, you could have a problem with new fixing brackets if the old ones are part of the mantle casting. Replacing 1) is impossible without having the mantle expertly cut off from the fireplace . Again, this might give you problems finding suitable fixing brackets.