The impossible to solve overheating problem

You said you adjusted the ignition from "very rich to lean" -- that doesn't make sense. The ignition controls advance and retard -- the carburetor controls rich to lean. So what exactly did you adjust?

Both extreme advance and extreme retard conditions can cause overheating, for different reasons to do with poor combustion.

That electric fan is large diameter but it doesn't cover a lot of the radiator (looks like less than 50%). Most set-ups use two slightly smaller fans for a wide radiator like that. Have you tried it with the stock engine-driven fan and no shroud?