Shelley was a key member of a close circle of visionary poets and writers that included Lord Byron; Leigh Hunt; Thomas Love Peacock; and his own second wife, Mary Shelley, the author of Frankenstein.
If the 900 watts means the input power, then 1000 w inverter would be ok. If the microwave produces 900 w of cooking power, you will need a bit more, maybe 1200-1400 w if you don't know what the input power rating of the microwave is.
As for what appliance will fit, no microwave is made which is only 15-16 inches heigh anymore. You will have to get one that is 16.5 so hopefully that doesn't create a problem for you but the replacement microwave that will work for you from GE is model JVM3160DF. We are on the honor system here.