If it is the original19.5 Hp Intek single B & S engine then it is covered by the Briggs OHV Singles manual
It might also be covered in more detail in the J D Technical manual.
I don't have one for the D series but all of the other J D Tech manuals I have include an engine repair section which is specific to the model engine fitted which is a lot easier to use then the B & S manual which covers 50 different variations so can be quite confusing.
Add to that you will also have the repair manual for the rest of the mower.
Digital download directly from J D is the cheapest usually
CD from dealers are sometimes a bit cheaper
Printed copy is very expensive .