if you had a ge spacemaker and need to replace it, last I looked they still make that model. if you know the width of your opening , the appropriate models could be located. Sharpusa .com would be a good place as far as the smaller to mid size and various features to review. even their very smallish models have turntables and preset buttons so its worth knowing what is available.

What are the dimensions (height, width, depth*) of the microwave cavity? Where is the electrical outlet for the oven, inside the cavity? Is the shelf above a range or cooktop (it shouldn't be)?

*of shelf