those original water heater doors are impossible to find, and the new water heater doors really don't look that great. if you don't have a replacement water heater, or are going electric, a simple patch panel would be a snap. if you have access to a shear, that makes a great, clean cut in the aluminum. i use one for all of my straight cuts. i'm planning on replacing the original water heater hole in my trotter with a louvered panel, if i can find someone to produce it for me. that's beyond my sheet-metal skills!
good luck! patch jobs are really rewarding when you're finished!
there's always room for one more!