One of the first things you’ll discover on the web when you set up a home machine shop is the Stirling Engine. These are ideal projects for model engineers of every level as they can be very simple or highly elaborate. The skills required making the engine are very similar to those required to make a model steam engine but you don’t need to make a boiler or worry about scalding yourself with leaking steam. Do beware however because all but the low temperature differential models will still get quite hot.
There’s plenty of information about making a stirling engine. There are a selection of films on youtube about how to make these miniature hot air engines
There are also a large selection of books on the topic.
There are also a multitude ofyour can make yourself.