I've built Ted's combination lock model and his model verifying Pythagoras' theorem model and can testify that they are very carefully built, easy to assemble, and work well when finished. They're great additions to your display cabinet and an easy way to explain the marvels of technology to the mechanical hoi polloi.
Ted, it would be exciting if you could offer a model engine that actually worked. If you're not already acquainted with Fluidyne engines and pumps, here are some urls to give you the idea.
https://hackaday.com/2011/03/09/simple-fluidyne-engine/
https://www.youtube.com/watch?v=7Gtins-mMRg
Those are all powered with torches but years ago I built a fluidyne engine sans pump using stacked plastic sheet and blocks very like your models employ. Cemented into this arrangement was a power resistor.
(https://www.use.com/images/s_2/1be283619976c66610a0_8.jpg) (https://www.use.com/1be283619976c66610a0?p=8)
(https://www.use.com/images/clicklarge3.gif) (https://www.use.com/1be283619976c66610a0?p=8)
(https://www.use.com/images/s_2/1be283619976c66610a0_9.jpg) (https://www.use.com/1be283619976c66610a0?p=9)
(https://www.use.com/images/clicklarge3.gif) (https://www.use.com/1be283619976c66610a0?p=9)
By heating this resistor with a current from an exterior power source I was able to obtain the pulsation that could be used to drive a simplistic water pump (basically two flap valves in series).
Give it a thought; if you make one, I'll buy it for sure.