Sizing of a chimney depends on btu firing capacity of boiler, fuel type, boiler design, and chimney height.
I doubt a 8" chimeny will work on a boiler with a 12" pipe connection.
Heatfab, Metalfab, Olympia all make metal chimeys 12" in dia. Most suppliers only stock 6 and 8 inch, so you will have to order 12" chimney.
Olympia will make chimney's out of 316 ss which is prefered for coal.
Look them up on the internet.http://www.olympiachimney.com/