I may have posted this info earlier, but, what y'all are describing is not a new idea. My Dad was an old time engine, collector. He was given a "Rider-Ericsson" HOT AIR engine.
We ran it many times. It had a 4" X 6" bore and stroke. Under the bottom of the engine, was a Can shaped device. Could have been a cylinder, you built a fire , or, in our case, put a hot water heaterburner in there, and ran on LPG. There was another "Cylinder"?? above the engine base, and that is where the valve mechanisms were. This unit weighed around 250 pounds, and was 5½' tall.
It ran a single bore piston water pump, pushing water up into a tank, above the roof of a house or barn. Output was 2 Gallons or so, per minute
I seriously doubt you could get more than 50 watts out of this rig. It had a flywheel that weighed, probably, 75 pounds or more.
I just ran a Google and them clicked "Images" on the upper left of the screen, and, there are several photos and Youtubes of those engines.
As a lot of y'all, I would LOVE to have one that runs off Solar, but, it would probably need water cooling on the cool side, to get sufficient power, IF a Fresnel lens was used to concentrate the suns heat on the bottom side.
My Son is searching TV repair shops, for a Projection TV. They have large Fresnels in them.