I would guess that as the Pepper's loco was built with components from a standard undertype engine of the period, this one would also use standard parts. If it helps to determine the component's used, the Hall's locomotive was standard gauge, with a single 9" x 12" cylinder, 5' 6" driving wheels, an overall length of 21' 6" and a wheelbase of 6'6".
Incidentally, Hall & Co. already had an 1867 Clayton & Shuttleworth traction engine locomotive, and in 1915 bought the second and final McLaren locomotive shown in this photo, an almost exact replica of the 1867 locomotive and the last non-Aveling & Porter loco.
**LINK**
As a model I suspect the performance of any traction engine loco would be a little underwhelming, as even a 7 1/4" gauge loco would only have the boiler of a 1 1/2" traction engine. One of the narrow-gauge Aveling geared 0-4-0 compounds might be a better bet, especially as there are several standard gauge locos in existence, as well as the remains of a narrow-gauge loco at Hollycombe.
Brian