I think compared to the replacement cost of the complete combo you will find the microwave repair cost reasonable.
Unless additional ventilation is provided to the rear and sides of your built in combo, a lack of circulating cooling air could have been at least part of the reason why your microwave failed - the electronics need lots of air circulation.
You might want to address this while the unit is removed for repair.
The voltages inside a microwave are potentially lethal and so the checking and repair is a specialised task...
Have to replace the inner door, some of those are available as a complete door assembly. Unit must come down and door unbolted from cavity of unit. Write down your model number, it is on the wall inside the unit. Take that number and call an appliance part store and give them that number, maybe look it up online and see what you find.