No, you need to use one that has the same specs as the old one. You should be able to find one online with the model number that's on the old one, or perhaps contacting the manufacturer for the part number. Circuits are designed with certain specs, that if altered, could cause poor performance (not as much power as before) or burn something else out. Digi-key is a good place to look for electronic parts too.
       
      
               
                     
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If you are within 5 to 10 % you will most certainly be fine. You will never find anyone who had a problem using a capacitor that was 0.05 uF different from the original. But certainly don't push it past 20%.
Despite some other comments, the capacitor is NOT part of any tuned oscillator working in conjunction with the magnetron. The magnetron is a self contained oscillator.
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