Returning to the question of whether the 800s can be converted to 21mm gauge. Paul Isles had this to say about it over on RMWeb .....
"The design is 21mm friendly, in that that all the bodywork, frames, valve chests etc are scaled to the prototype, and with the wheel sets being 00, there is room to 'expand'. The con rods etc have been spaced out from the wheels in an effort to maintain the look of the prototype, while ensuring that they operate reliably, but this will be subject to further testing. The EP that was running at Malahide was the very first example provided by the factory, and has been updated since (although the updated version is with ESU for PCB design).
HOWEVER, Finescale 21mm modelling is even more of a niche in Ireland than it is in the UK, so any 21mm friendly attributes are there purely to give the right appearance to the locomotive, rather than to directly accommodate 21mm modelling. If that's your 'thing' then I will wish you luck, and look forward to seeing what you do with the model."