Hello Andrew,
If you were inclined to build a steam engine similar to polly, i would suggest a Sweet Pea
Which is a Narrow gauge Bagnall style loco in 5″ gauge. It has relatively few parts is easy to build and produces a fairly powerful engine which can do fair work.
Their is a build series book in paperback by Jack Buckler called “Build your own steam locomotives. It is upto date and the annotation and diagrams are easy to read, and the text is step by step.
The main distributer is “Blackgates enginering” you can use them for as much or as little from full kit, to basic castings if you want to do it all your self.
There are many pictures on the net of these engines and nobody every speaks badly of them, they are proved and reliable engine. If you interested, always youtube “sweet pea rally” as there is one every year.
kind regards,
Kieran