There must be as many ways of approaching a machining job as there are machinists. Edgar T Westbury provides his workholding ideas and construction sequence in his 1947 book on building the Atom Minor Mk III. These pages show how Jan Huning (UK) went about the tasks. Jan used castings made from the old "Woking Precision" patterns. While perfectly servicable, the thing they rather lack is—you guessed it—precision! Still, as we'll see on these pages, that does not stop the competent model engineer from building a fine running engine from them.
The links below will take you pages with photos and text provided by Jan describing how he machined each part. Apart from his rotary table with "T" slots, Jan has not used any exotic equipment. These should give you plenty of ideas of how to achieve the same ends on the equipment you have available.
Main Bearing Housing
Rotary Disc Valve
The Connecting Rod
Please submit all questions and comments to firstname.lastname@example.org