Clean ? Not yet, in your latest pics anyhow..I see NO new pics ? And that is critical. Someone never took the time to keep the engine clean as it should be. If it was kept clean, all the disassembly would not be necessary.
And to why the fins have metal cover plates around them, that is to direct the air coming down from the blower (round thing on top of engine with the fins sticking up & rotates when running), the air must be contained so it can be directed to blow over the engine, thus cooling it. It yours was liquid cooled no cover plates would be necessary.
Check the spark plug, it 'should' have a tan to light brown color to it.
As to your solid bad, well, how about finding a hollow bar that is just slightly bigger & put it over the 2 sections.
The muffler: Can you look inside it & see if some critter hasn't built a nest in there, a partially blocked muffler will cause overheat as well.