Your magnetron is practically unkillable. It's more likely to be the "doser" that's failed. I've got a few microwaves working for myself at a bit better than their plated certification by just removing the doser and letting the mag run free. There's no real danger because the enclosure is complete, or rather it should be. The only failure I've had so far was a Fagor combined oven/microwave that leaked so badly afterwards that an ordinary multimeter needle rocked over to full deflection and back in synch with the test load rotating on the turntable. It probably leaked before it received my ministrations, but it went to the local recycling centre anyway. Incidentally, I wouldn't have thought that an ordinary multimeter (with no leads attached) could detect microwave leakage with the output being in the Gigs. It must be down to harmonics or some unexpected effect within the diode or something, but you could try that yourself. The deflection was still 100% right across the room (on any volt range AC or DC) and was very directional. It left me wondering how many people impatient for their soup in the works canteen at lunchtime or whatever hover about right in front of many leaky microwaves getting a powerful beam through their stomachs.
Time will tell.
Des.