Havoc,
the real one was meter gauige, so 1:22.5 on 45mm would be prototypical.
He changed the gauge from meter to 600mm in order to fit with the Decauvile locomotive he has also plans for. So his "scale" isn't 22.3 but 13.3! That's why it is so massive.
I agree and think the boiler could be made as a simple center flue and it would steam fine
I would put in a few tubes. Just a central flue for the superheater and a ring around it. Far easier than the sloped and bended ones. It might not need to run fast but with the gearing the engine will run rather fast. And the engine has a 10mm bore by 20mm stroke (0.4"x0.79" for those still in the middle ages).
Before anyone starts this model: take a ruler and try to visualise it! It is huge... For those not used to mm, it is almost 2 feet long, more than half a feet high. 2/3' high.