We repair a great number of built in microwave ovens. For many years now new kitchens have had the option to have a microwave or combi oven fitted within the wall units to keep the worktop clear.
The microwave tends to be based around a free standing microwave so from our point of view will be just as easily repaired – once it is out of the unit. To that end if you can remove it from the housing why not bring it into our shop to save the call out. The repair will be even cheaper.
The unit often has the trim pieces top and bottom fitted to the oven. if you can see screws then they are probably just locating the oven within the housing. It is usually sat on a plynth to centralise it in the housing and more often than not held in mainly by gravity.
Like a lot of built in appliances, it will be a lot easier and no doubt cheaper to repair the microwave than to have to find a replacement that is the same design and size.
As you can see from the photos here of a typical example that has come in for repair. The trim is fitted to the oven so has come out of the housing with it and there are even hand holds for carrying the oven to your car.