I have made some further progress, boilers are complete and wheels attached:
I will say, for anyone attempting the build, I found the boiler internal supports and spacers a tad confusing. They are numbered 1 through 14 and are glued to thicker card before using. I assumed these were intended to be placed in that same order. They are not however.
1-6 are internal to the main boiler sections (there are two, that get joined).
#7 and #10 are internal (and on edge) to the foremost boiler section (the slightly larger one). #8, 9 are stacked on #7 and used to join the two differently sized boiler sections, with the textured side facing the rear.
Numbers 11-14 are stacked at the end of the larger boiler section.
Marcin, I'll snap a shot against my 30cm ruler for you. The only dimension I know for certain is the inside wheel base, which is 1 1/4" or 31.75 mm. The actual wheel centers are closer to 33-34 mm. It will never sit on real track, so whatever I make to suit it will fit the slight gauge offset.