The Goal: Scratch-Built Dewey Bros. Locomotive

For the next year I’ve set an ambitious goal: I’d like to attempt my very first scratch-build of a steam locomotive in Fn3 (1/20.32, 3-ft-narrow gauge).
My model of choice is an industrial loco of the late 19th century:
Picture of the prototype with some measures
(Source: Sinclair, Angus: Development of the locomotive engine, New York 1907, p.511)