Whilst I was researching Sentinel boilers, I looked at a variety of different Sentinel road vehicles on both road and (standard gauge) rail. The boilers varied somewhat throughout the years. I had to guess which of the various ones I looked at was closest to the features I could see on photographs of this type of locomotive (since I couldn't see the real thing). However certain elements of the boilers seemed to remain the same: the top mounted fire door, the side mounted regulator and valves, the safety valve mounted in front of the chimney with a pipe running up in front of it and the basic dimensions. From that I made my model.
The cylinder assembly probably did vary because of the different drive between loco and road although that didn't bother me for, as far as I was concerned, a chain drive appeared from the base of the cylinder housing and drove on the front axle. I didn't have to model the cylinders.
For me, the major compromise was to fit chain guards throughout. From contemporary photos, Most locos didn't have them on both chains. I did try to find a small enough chain at the time and was told politely by a number of chain manufacturers that no such thing existed (it may now of course).
More recent research or greater knowledge may prove some of my suppositions wrong and I'm open to correction because of that. For example, Roy Link's drawing wasn't available when I built my model. However it is dimensionally very similar to the Wrightlines kit and I think that was built from the drawing.
And by the way, I don't think that, as it stands, the Cliffe Hill example could be regauged to 15 inches. It is a large machine and Sentinel boilers are all pretty much the same size. However, an 18 inch gauge Sentinel(ish) machine did exist on the Jaywick Miniature Railway but based on a different narrow gauge type. The remains of this are currently in the safe hands of a miniature railway enthusiast so it may steam again one day.