based on drawings for a working demonstration of a Root's steam engine. I'm not sure exactly how the crank will turn out, but different production methods should be able to be used to reproduce this model once completed.
the models are "drilled" but not tapped.

The OpenSCAD source is included.

sizes from the drawings are:
Cover to Valve cover setscrews, tap #4-40x 0.19" deep TYP x 8
Exhaust ports, tap .25-28 NF x 0.25" deep TYP x 2
Pressure port, tap .25-28 NF x 0.25" deep TYP x 1
Valve Cover to Body set bolts, tap #6-32 x 0.31" TYP x 12
Valve to Valve Shaft grub screw, #4-40 TYP x1

The only component not shown in these STL files is the SPACER.
OD 0.5" x ID 0.32" thickness to suit.
Not yet assembled to verify what size or thickness this should be to complete the model.

Added 20141207
spacer, which is a piece that sits between the valve cover and the valve to provide thrust support on the side of the valve. This will need a slight adjustment to tune to your model.
valve lockring, which is due to the set screw hole in the valve being impractical for securing the vale to the valve shaft.
The valve locking uses m3 nut traps and an m3 setscrew to lock the valve shaft to the valve at the appropriate angle to time the engine.

drawings taken from Steam & Stirling Engines You Can Build Book 2 - A compilation of engines from Live Steam between 1982 and 1993.