Its much easier than you might think.
How to replace swamp cooler with attic fan.
Evap swamp coolers are great for dry climates.
If you choose not to remove the ductwork you could install a large attic fan in the roof void.
It is best to remove the ductwork from your attic when you remove a swamp cooler from the roof.
How to make an evaporative air cooler out of a box fan.
This is due to the development of condensation within the ducts leading to mold growth.