The side of my microwave set on fire
If anything that is actually part of the microwave caught fire, of course you should replace it.
(Theoretically you could have it repaired by someone knowledgeable, but repairing a microwave is not cost effective.)

Continuing to use damaged electrical equipment is asking for a fire or damage to your electrical systems.

There is no such thing as a 'one off' fire that you can neglect.