If it’s a fairly ‘old’ Thornycroft then it might be a Ford ‘Dover’ series engine 6-cyclinder - 6 litre also, (and still being) marinised by Sabre, now with turbo-chargers, the original was rated from 96-108HP unblown. It was never fitted to any road-going vehicles, other than combine-harvesters. Mostly it was designed as a fixed stationary engine and is still used today as emergency fire-pumps in building and for large power generator sets.
When I was looking for a manual, someone asked “Why”? “Nothing ever goes wrong with ‘em” and several years later, ‘twas still true.
There is a Thornycroft website, but it deals only with the later Perkins derivatives, I think.
However, ASAP Supplies Ltd specialise in spares for the Thornycroft/Ford engines and will probably sell you a manual.