Hi Patrick. Great plan. Have you got David Lloyd’s book on modelling the Irish NG? Includes pictures of his work on an Antrim tank. Re parts, S scale Society may have various LMS loco castings that would work, or some 4mm ones may be suitable. Bill Scott’s book on NCC locos has drawings which some legerdemain with a photocopier should produce something helpful for you. Re valve gear, a bit of trial and error should produce something acceptable. If you build up the cylinders a bit and add a bit of plastic bar and rod in likely places, you can give a fair impression of what should be there. The S and S1 are tricky beasts for modelling (they don’t have a traditional footplate as a foundation) so if you can produce something that’s largely correct as your first effort, you can be pleased enough!