I'v been out of the workshop for a few months with a crook leg, and am just getting back to it, so I started with something simple, a weather vane, in the form of a Cessna 185, this was the main aircraft type that I worked on, years ago. Now I'm getting back to my hot air / Stirling Engines. I don't, but you could, just to find out how they work, build one of the tin can hot air engines that you can find on the net, you then might like to build a machined version, or go to a steam/compressed air engine, plans of which may also be found on google.
I started modeling by making tap wrenches, and die holders, some for sale, some to keep, and before making any models, started making parts for vintage cars, and tractors. Ian S C