Ours packed up last year, and I took it to a guy in Crosby. I retired to a local Pub
for a pint and read the paper, by which time he'd replaced the magnetron. £50 and it has been fine ever since. Magnetrons are a form of thermionic valve and they don't last forever, but you don't need to buy a complete microwave. The Magnetron can be replaced.
Next time I'll buy a magnetron online and fit it myself. They are only £35, and it's not hard to do. You have to be careful of damaging the ceramic insulators though as they contain beryllium oxide. This is poisonous and may be mildly radioactive. It's fine as long as you leave it alone but you really don't want to inhale or ingest the dust if it gets damaged. Just be careful.
One tip, by the way, is to make sure the air intake grilles don't become choked with dust. they are usually behind the thing, so unless you dig it out you won't know until the thing overheats and dies on you. I haul our microwave out and run the Vacuum over it from time to time.