Uhmm, this is a tough situation.
I'm an Electrical Engineer and I know exactly what is needed to work on those; but as the previous commenter said, these are low-priced consumables.
Do the 'external stuff' first: check the outlets, fuses and circuit breakers associated with the circuit branch.
Then, I am going to give you some warnings:
First, these are NOT SERVICEABLE BY THE USER, unless you have considerable ELECTRONIC experience.
You will see a sticker/placard to that effect on the covers you have to remove. 'They' mean it and 'they' are DEAD serious.
Electrical, mechanic and 'handyman' experience doesn't count: the only moving parts are the fans. There are no 'intuitive' cues that the normally mechanically inclined person can use to figure these things out.
There is usually a simple but decent schematic pasted on a cover. This is for the service person, although they usually have their own books. This diagram is not meaningful unless you have extensive experience, as microwave ovens have a few 'shortcuts' in their design that you need to be familiar with. Those shortcuts keep the cost down but will surprise even a person with the proper credentials the first time they encounter them.
And: THE VOLTAGES PRESENT ARE VERY HAZARDOUS!!!! LETHAL is the term used here, but DEADLY will do as well.
Next, these things are small TRANSMITTERS; small but high powered, usually a kilowatt or so. That accounts for part of their danger.
The Transmitter aspect also means that you can do something illegal that will cause undue interference from radiation effects. Technically, you should be licensed to work on these things and you need extremely specialized instruments to check them out.
Radiation effects bring up other safety concerns: leaving a shield off can produce a hazard. Also, looking into a microwave feedhorn with power engaged can cause eye damage. The RF radiation here is invisible, but if you feel the 'infrared' heating effect, it is too late. The repair bill is preferable to eye surgery.
Ordinarily, Gregor's advice of 'throw it away and get another one' is best here; but you have built-in. You are stuck with the particular mechanics of your installation and lack flexibility. that limits your options and could make this expensive.
So, there are three choices here:
Call the Factory Service guy. This is the best in the long run.
Abandon the 'built-in' and get a 'general purpose' (undedicated) unit and use it elsewhere. They are cheap enough to consider this, especially for a temporary fix.
If your unit is 'no longer supported', get together with a good cabinet maker and pick out a current model. Have the cabinet maker do what he has to do to install your new selection. He should be able to do this without having to gut the unit. Avoid removing the exterior metal covers, even if you bury this thing in wood: THE RF RADIATION FROM THE INTERIOR WILL GO RIGHT THROUGH WOOD WITH NO ATTENUATION.
If you have to go this route, pick a major brand and tell them what you are up to; the idea here is to get something that will be supported as long as possible. The dealers may even have a model made specifically for this service.
And that last paragraph is your best bet: start with the dealers and tell them exactly your problem.
PS: "Magatron" is Magnetron. And yes, that is the most likely source of trouble, if there is significant age here; but Power Supply components could be gone as well.
PS2:
"It's the same reason you no longer see TV repair business, VCR repair, lawn mower repair. etc. etc. The labor and parts necessary for the repairs are often times equal to or greater
than the cost of a replacement unit. "
This is known in the industry as "Beyond Economical Repair"; BER for short.
