Hallo Derek,
I don't know what the previous 'poster' said but since it was deleted I can imagine. On a more positive note (and from an entirely different perspective) there are a number of very nice B/L Scotsman out there still and some can regularly be seen running extremely well at Gauge '3' Society GTGs – where (with a long rake of teak coaches) they make a most impressive sight.
All of the versions I've seen (I think) have had meths boilers, so (in my view) these are locomotives more suitable for scenic use (Gauge 3) than passenger hauling (the root of the removed comments I suspect). However, if replacing the boiler, then a coal fired one could be substituted of course. I've not looked at the detail but the boiler from the later 2.5" ME Collins version of the Scotsman could possibly be modified to suit (probably need slightly reducing in size). I have one if you would like some general dimensions to think about and also the associated ME build articles (PM me if you are interested).
However, if you intend to just 'display' the engine then this may be academic (and in which case why not just use a dummy boiler – much easier and cheaper) – although the engines value (both financial and aesthetic) would be greatly enhanced if it was useable.
Finally, the National 2.5" Association may well be able to help with original plans and parts. I know there is a 'build' booklet available but I'm also sure there will be B/L plans out there somewhere with the membership. There are hundreds of members of both the N2.5A and the G3 Society who very much enjoy working in 2.5", who understand the benefits of the 'smaller' gauge (lower weight, lower costs, usually simpler to build) and who get a great deal of enjoyment in doing so – myself included.
So welcome to the 2.5" family! 
Regards,
IanT