As noted above, working inside a microwave is dangerous, even when turned-off and
unplugged, because of capacitors inside retaining their voltage.
No, microwave ovens come in all sizes. If you replace it, you need to measure the
space you have available, to make sure the one you buy will fit in the space available.
I wouldn't try to fix it if you have no electronics background. If you don't know anybody
who does know how to work on a microwave oven, I'd just replace it. They're not all
that expensive these days.
Good luck with whatever you decide.