Mucked around with sounds like serious understatement, especially considering the rust muck left inside of it all these years. I'm sure you are right about him dropping something ferrous into the boiler as I can see no other way for that rich thick rust muck to have formed!
It will require a whole lot of luck for that red mud not to have gotten into the steam pipe and cylinder, but perhaps you are over due for a little luck on this project!?!?