Hi Robin
Can I suggest you join the National 21/2″ gauge association or at least have a look at their website, very helpful and friendly, with a wealth of experience and they can supply castings and drawing reprints, may even have some original castings available. There are many of these unfinished about and parts do come up for sale regularly, quite a few members have finished very ancient part builds of these engines so advice would be available. someone is bound to have some parts to help you out, quite recently I gave a set of B/L Scotsman cylinder castings to an advertiser in the club newsletter who was in a similar position to yourself, parts are out there. Good luck with your project, for extra inspiration have a look at the excellent website of John Baguley, serial rebuilder of 21/2″ gauge engines, found the answers to many problems there.
Dave