Freelance engines have to look the part. As though they should work.
I've seen too many freelance "Garratts" that miss the point of the engine, a big firebox unobstructed by frames, wheels and axles, and a short fat boiler. Often using eg two G-gauge Colorado narrow gauge engines. They simply don't look right because they retain the original boiler and firebox. Not to mention the bar frames.
Patentees are another matter. Anybody interested in them will have seen plenty of drawings by Ahrons, Sharman etc. Even North Star or Arend. I bought a set in a used bookshop in Palo Alto in the 1990s which were reproduced from a Victorian reference, and so detailed you could almost build the real thing.
At that time it was like the American scene a few decades later, with most companies buying off the shelf engines. I happen to like the E.B.Wilson engines of that era - the Jenny Linds and the corresponding
2-4-0s and 0-6-0s as well as the small tank engines that were the precursors of the standard early Manning Wardles.