Replacing the water heater is not hard. You need to be sure to get the right model that has engine heat as an option.
The new one should fit into the old hole without any cutting or modification in the skin. SAve the old door as it should fit and you would have to order the door from Airstream if you want an aluminum one. The one that comes on the heater will be white.
I have bought 2 heaters for diffrent MH from http://www.rvcatalog.com
They seem to have the best prices and freight is free. Here is the link to the atwood page
You may want to call them as the only one with the heat exchanger is also a 120 volt model. It does have DSI ignition too for gas. You could install it and not use the 120 volt option and still save money on the unit from them.