Model engine kits are readily available, I bought this Crosskills engine kit at a model engineering show.  The kit comes supplied as castings and various steel components for machining.  Full engineering drawings are supplied too.  The design is by Anthony Mount a respected kit maker and the  finished model is only about 6 inches high, the brass flywheel is about 3 inches in diameter.

All the parts for the model come supplied as raw material such as brass, mild steel and castings.  Drawings are supplied too.

I machined the model on a Hobbymat lathe with an integral milling head attached to the lathe bed

Machining the pedestal base

Machining the brass flywheel

Turning the four pedestal support pillars from square bar.  A four jaw chuck is essential for operations like this.

Forming the crank.  The central bar will be cut out after soldering the pieces together.

Crankshaft completed.

Steam chest showing the drilled ports blanked off with brass screws which will be solderd into place

Completed steam chest

Machining the piston barrel on a mandrel  for concentricity and to ensure the faces are true to the bore.

Test assembly

Crosskills Engine assembled and completed