I asked a friend of mine , who is a certifyed boiler repair and builder, about a replacement for the 9's boiler he quoted me between $200-280,000 for a replacement boiler for her demensions.
If you want to use her in daily service then a new boiler is going to save you a ton of cash in not having to do the annual inspections for recertification of a lap seam boiler and a lot of headaches in keeping an ageing boiler happy. On the down side you do lose a big piece of the original locomotive. I know there are only , perhaps 3 or 4 bits of her from 1884 left after her rebuilding in the teens but when you are dealing with preservation you do have to ask yourself at what point to you want to replace things before you end up with "Grandpas Axe". she is already grandpa's axe in that she has an EXO boiler, a rebuilt frame, a new tender, cab, pilot , cylinder castings , drivers and tires so she isnt the original engine that came from Cooks to begin with so woudl a new boiler add to that or not?
but i still say you cant preserve somethign if you run it all the time. If they want to run the 9 , then by all means do so , but I recommend they take their cue from Dan Markoff and treat her with the respect that he shows the Eureka and run her once in a while for special occasions instead of runnign her all the time becasue you have no choice.