I had to replace a microwave also. The replacement is easy-just about any m/w will work. the problem is the trim plate that goes around it. The cabinet space is much larger than the m/w, so there is a decorative frame around the original m/w that has the same outer dimension as the outside of the cabinet and the same cutout as the outside of the m/w.if you don't mind the appearance, you can gain some storage space by simply installing the m/w completely to one side of the cabinet and have the balance of the cabinet space open. You may have enough room to add a shelf over the m/w or mount the m/w on a shelf and have open space below the m/w. Secure the m/w with wood trim all around the bottom so that it cannot move side to side or front to back. Make sure you leave enough side and front clearance for the door to open completely. Note where the receptacle is located before you decide which is the best right/left or top/bottom location for the m/w as they all differ from pod to pod and m/w to m/w.
hope this helps
Vann