I would imagine that the keypad could be retained but power removed from it. Removing the microwave's fascia with the rivets or "buttons" would be my first step. You may find that a sheet of metal needs to be installed under the burners if the oven is enclosed and the top acts as the burner drip pan.
It may entail some work but the microwave should be replacable. If the function of the existing microwave has only failed due to keypad failure or some other electrical failure, it's possible that it can be repaired. Unfortunately, with the higher cost of repair compared to the low cost of a replacement, replacement may be the more attractive method.