There is a microwave repair shop off auburn blvd. in Sacramento, CA that I spoke to about microwave repair.
They told me that they pretty much do commercial exclusively.
This is because the home micros are too cheap to repair - you just replace it.
Messing around with this, it has been my (limited) experience that stuff like the fan motors, capacitors, turntables, turntable motors, magnetrons, etc. are pretty generic, or at least are swappable.
It seems that the effort for servicing microwaves is best applied to commercial. you'd be surprised how many restaurants use microwaves, up to and including the very highest end gourmet restaurants.
Microwaves heat the moisture w/in the food. They don't "cook" as we understand the term. This means that, with limited application, they are indispensible for heating and reheating.
After all, nobody cooks their turkey with a microwave. but they sure do heat it up with one