Ah – I responded then your post appeared on my computer just as I did so!
You’ve answered the basic question I had asked, and this shows you and I are faced with a somewhat similar difficulty, so I have re-written my message.
Common details like the lubricator, motion-work etc. have of course to be designed specifically for the locomotive but a book like that by Martin Evans will help you do that by showing their basics. Really, it’s a matter of collecting all the information you can!
I have similar problems because as my main project is a steam-wagon built from no drawings at all apart from a patent specification, just old publicity photographs, basic descriptions and sparse dimensions given in the manufacturer’s catalogue and trade-magazine reviews published more than 100 years ago.
So to try to design the model I have to use a combination of old “full-size” engineering, and modern model-engineering, text-books – including Martin Evans’ book on model locomotive building.