Once again to all, Thankyou for your input.
OK, agree with the cannon needing a steel liner. In my day job, I work for a Non-Destructive Testing & Engineering company and have provided many metallurgical tests on casting of all sorts. This is why I was worried about the cast iron in the first place but here is the deal. The guy who is selling this cannon is asking 400 bucks for it. I my searches on the internet 400 bucks will not get you anything that even looks as good as this old cannon does nor its size. I am proficient at welding and fabrication so I can create the liner at my home, but to install it properly I would have to bore out the original cannon a little bit to fit the steel liner. I was worried that I would be destroying or at the least modifying an a cannon that had intrinsic value, but from what you guys are telling me I shouldn't worry about it because it was a rather common cannon sold as a novelty in the late 1800's.
So it sounds like to me that if I bought it and modified it with a steel liner I would have a very nice, very shootable, unique cannon, unless you guys thing it is a waste of time.