The RCTS book on the standard pacifics is a great starting point. Not all got speedos, there are minimum of three smoke deflector types - original with a handrail / western with brass edged cut-outs / wcml dished hand-grabs - then there are different connecting rods either plain or fluted, depending on when built. I'd try and hunt down 70034 Thomas Hardy for a later GE loco, though the Hornby model has the early crest and no AWS box on the running plate.
Ian