This is very nice looking work! It is funny that without your measurements, it would be difficult to determine the actual size of this engine. I do have a few questions about your build...
What solder are you using, and what method of application? I.E. Torch, oven, etc..
It appears all drivers are flanged, are you incorporating any type of lateral motion in the drivers? Which drivers, and how much clearance for each?
What will the boiler and tubes be constructed from, and what will the operating pressure be?
Sorry for the questions, my brother and I are considering a similar project sometime later down the road.
Thanks!
Casey