Ed Bertschy provides a template that can be downloaded and printed on good paper to be used in the construction of a paper steam engine.
This paper steam engine is based on a Riches and Watts nominal 2 1/2 horsepower vertical A-frame double acting simple slide-valve steam engine circa 1870-75. The original engine was used to drive a water pump to irrigate the fields of Norfolk. If this model had a scale, it would be roughly 1:19. I had to double the size of the eccentric and strap in order to make it buildable. Everything else is close to scale, but changed in design and appearance because, well, paper isn't iron.
Everything will work on this engine, the flywheel turns, there really is a slide valve in the valve chest, the piston works, the cranks and the connecting rods, eccentric and strap all work if built carefully. The final model stands about 12 inches high. I hope you have as much fun building it as I had designing it.