With your model number, I would check this GE bulletin first!
http://www.microwavedisplay.com/ge_mw_mag_kit_bulletin.pdf
GE had a run of units whose magnetrons fail prematurely and they have a campaign (not a recall, mind you!) under which they supply a replacement stirrer, magnetron, and high-votlage rectifier diode.
Once you've looked at the bulletin, let us know if we can help further.
William E Miller † MicrowaveDisplay.com, March 2007