Hi Richard,, other than the thimble what issues does the chimney have?? If I remember, you don't have any draft problems.
I'd follow the suggestion above and put in a terracotta thimble,, just mortar it in,, and replace the cleanout door,, they are cheap.. Just paint it with good enamel or Rust-Oleum paint [a very good enamel] before installing it... It will last a long time..
With only a few repairs to do,, I'd save the $$ and spend it on a coal bin or another load of coal before the prices go higher..