A number of changes, mostly invisible. I'm not one of the guys funding and directing the project, but a couple differences I'm aware of (in addition to using diesel to fuel the boiler), are: heavier gauge steel in places for more weight, and more modern materials in some places (example: stainless steel for the water tank). Fuel choice is for fire safety (who wants to see a modern, super-ugly cinder catcher on an 1880s 4-4-0 which had a diamond stack in period? - and nearby houses) convenience (no active coal mines or distributors within 100 miles, gas station across the street, and you can shut it down quickly after demonstrations with no waste to dispose of), and environmental (less of a concern, but a real one in this area these days - waste disposal, storage, etc).
FRA is not the main reason for a possible spring delivery - more on that soon, I hope. Patience, all.