Replace the drive motor...
http://www.repairclinic.com/referral.asp?R=153&N=769741
Drive motor, glass cooking tray.

Fairly easy to change, remove power, remove the bottom panals screws to
access the motor.

jeff.
Appliance Repair Aid
http://www.applianceaid.com/

Perhaps! The motor that rotates is basically a simple AC motor etc.
Design of m.wave 'may' make it easy to just get at the bottom of the unit?
Maybe not?
And if you are not electronics capable; two very serious warnings.
1) There is some 6000 volts DC around the power supply and magnetron. That
6000 volts is some 500 to 1000 watts of electrical energy which is
potentially lethal if 'one finger' is in the wrong place.
2) It is a microwave radio transmitter producing some 500 or more watts of
radiation (that's what cooks the food in minutes or seconds). Do not stand
near it or operate it when case is open. Make sure it is correctly closed up
with all the RF (Radio Frequency) sealing edges properly meshed, screws back
in place etc. Best to use one of those cheap m.wave radiation meters
(About$15) to ensure radiation is not leaking out around the door or the
edges. Such radiation could affect (i.e. cook or cause cancer) to people,
especially children, who pass nearby and/or are more of suitable height to
be exposed to m.wave radiation at counter top/table height!
Please be very careful.