There is also a section on the system in "Railway Holiday in Northern Germany" (WJK Davies/D&C, 1965ish) but it doesn't cover Werl.
Davies's book, which I have -- published indeed in 1965 -- tells of a tour which he made, I think, the previous year. By inference from the several pages in the book, about the Ruhr-Lippe-Kleinbahn; plus material from the item linked-to by the OP: as at 1964, the only parts of the system then remaining in use were two standard-gauge sections, freight-only, respectively at its northern and southern extremities -- on the more northerly and longer of which (some 20km. running due east from Hamm), Davies travelled by special permission on a freight. The linked-to item tells of end of passenger service on the route through Werl in 1952 (together with that on most of the system): complete closure east of Werl in 1953, and north-west of Werl in 1962.
The linked-to item claims that as at 1936, with system at maximum extent: [approximately-rounded figures] of a total of 100 km., 25 km. were standard gauge, 20 km. three-rail, and 55 km. metre-gauge only. (For what it's worth, Davies gives different figures: one would be inclined, I feel, to give greater credence to the directly-German source.) Neither source gives a fully detailed breakdown of what was the gauge status re the entire system: it can be ascertained, though, that there were standard-gauge / mixed parts at system's southern end, quite close to Werl; I feel that it could be hypothesised that there might have been mixed gauge into Werl from the south and east?