Stew,
When I was a teenager and used to go to all the steam fairs without fail, I used to talk a lot to a chap that had built one. The only scale I've seen them in is 2" to the ft. Small but surprisingly powerful little engine. He let me drive it a few times, actually I still see some of the old faces around when I go to these things but not sure whether they'd remember me! He made a 3" scale fowler ploughing engine after that.
Nick