Repair the chimney or replace part of it
A national seismic retrofit plan set lays out a few ways to handle a damaged brick chimney. You can cap it at the roof or ceiling, replace it above the shoulder, where it narrows over the firebox, on top of a masonry firebox or a factory insert, or replace both firebox and chimney with a factory insert and a metal flue. If your firebox is sound, I lean toward keeping it and rebuilding only what's failed, because you keep more of the brick your house was built with.

