Buy two new manual doors,
Mount them and check for fittment.
Your Done, hehe
Going from power to manual would be a pain.
You would basicly be gutting them, and putting in new gears and Everything.
The door panels would need holes and the switch plates covered or replaced with something else.
I know its a real basic answer, and not in depth. but trading out the doors would sereously be the easiest way.
If nothing else, you would be able to sell the old ones and probably make more then what you paid for the manual ones.