Lee Valley used to sell a toy boat with a "steam engine". It consists of a boiler and two pipes sticking out the back of the boat. A candle was placed under the rpimed boiler, and then the boat was set in the water. Once the boiler heated, the steam was expelled out the back, pushing the boat forward. The vacuum created would then draw more water into the boiler. The boat makes a great put-put sound.
I don't know how this might help you, but it is the smallest steam engine I have seen...
You might try their web site -
www.leevalley.com . I did not find it, but sometimes their search engine works in mysterious ways.
Andrew