crazy idea

aluminum alloy is OK to use as a head or piston material, I cannot see why it cannot be used for exhaust manifolds.

pistons whether 2 cycle or 4 cycle get cooling from the incoming fuel - air charge as well as from the lubricating oil,
many performance engines and most diesels have an additional oil jet that squirts oil on the bottom side of the piston to cool it.
and remember, it is only the top of the piston that gets exposed to the combustion temperature, the skirts and under crown areas
absorb the heat from the piston top and let the heat out to the rings and cylinder walls, but they do not get the direct combustion heat.

cylinder heads, air or water cooled have provisions to remove heat, the water cooled heads have internal passages.
The air cooled heads will have fins to increase the surface air for the heat to dissipate to.

So certainly an aluminum exhaust manifold could be made, but for the manifold to have any realistic durability factor it would need
1) substantial increase in wall thickness
2) provisions for cooling, either a water jacket or cooling fins.