The article does have quite a bit of detail, enough to recreate a set of plans.
So what I have decided to do, or at least try to, is to build a 3D model in Sketchup.

Sounds like a great idea, but nothing beats actually building one. Which also sounds like another great idea.

An even better idea is to build a 1/4 scale model. That way I can make as many mistakes as I need to without wasting too much of my precious stash of timber (and other useful things.)

I have started with the basics - I found an empty tin can of just the right size to represent the boiler.

For the wheels, I am making simple wooden disks without the flange. No point in spending too much time and effort in making too much detail at this stage.