Yes they can be replaced if you can find the correct one.
Some of them just clip in others require taking the unit apart.
As a test I would just remove it and see if it still arcs.
If it requires taking the unit apart I would just carefully cut it and remove it from inside the oven.
This is just for testing, do not run the unit like this.
Try heating up some water with a loose cover do not let it come to a full boil.
If it still arcs then either the magnetron or it's control electronics are bad and I would replace the unit.
If it does not arc then hunt up a replacement and install it.
If you have to take the unit apart be very, very careful.
Many older units do not discharge the high voltage when the unit is off/unplugged so you can get a nasty shock or worse.
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