Hi and welcome,
I have attached a photo of another Thornycroft Nubian, which you probably know off . The manuals for the engine would be Rolls Royce B range, this cover 4, 6 and 8 cyl engines, the official RR manual is often seen in original or copied, also the army produced their version (the better option if you get the complete works), in EMER Power form. The engines were used in a lot of military vehicles as well as civilian roles.
regards, Richard