As illustrated in the second image, improvements have come a long way, smaller size and better cooling and regenerating of the gas in a closed system, achieve greater efficiency. The most advanced units are being used for special applications, like NASA's space program. I have pictures of them somewhere.
So I went there because the first project is a light hot air engine that will do a steady 400 rpm by simply burning a few scraps of wood from the shop. It is made from mostly light tin containers and other metal scraps from around the shop. As the article says tin snips and a soldering iron are your main tools. This is a nice project for the hobbyist and beginning model maker.
For more advanced projects Dave Gingery built and wrote a few books on different engine types. Heres a picture of his Stirling Cycle Engine manual.
The next project is a nice hand grinder (like the Dremel) one-tool shop. This is ideal for the hobbyist with a small shop or an apartment dweller with a small closet or spare room shop who builds models or other small projects. It will perform many operations without the noise, flying dust and expense.
Every drill press should come with the next project pre-installed. A foot feed for a drill press is very handy for many operations, freeing up both hands for the work being done. Mortising on the drill press comes to mind.