Mostly Stephenson valve gear was used for this, but then four excentrics on the main shaft are required. Using lego it would result in a weak shaft. The Maudsley gear used on this engine avoids this , also it makes it very easy to adjust the valve timing.
These types of files are created with the LDraw system of software. You will need to install compatible software to open and view the instructions.
A full list of available software is on the LDraw site.
This MOC also has a LEGO Digital Designer file available: 3D Model