What parts do you need? Engine parts manuals can be specific to the application and harder to come by because engine attachments alt, starter, filters ect are manufacture specific. A simple google search came up with this but for a specific application. You may have to find manuals through the dealer.
https://svseeker.com/files/5.9 Cummins Manual-AERA Diesel parts.pdf
The first link has manuals for multiple engines manufacturers. For instance I access to kubota engine manuals but a kubota engine in a takeuchi wont come up in The industrial engine side of kubota. The same engine is in there but not specific to that engine.
Maybe theres others on here with more perkins and cummins experience. I'm trained for perkins but because its through a certain manufacture I only have access to a limited amount of engines