Welcome to the forum!
For the stripped screws, you can try wrapping some plumbers' Teflon tape around the threads to build them up enough that they will hold.
The second not so easy option would be to fill in the holes in the shell with epoxy, and drill the holes again. The screws are #6 self tapping, fillister head. Lining up the drill properly centered to drill the holes is the hard part.
The next option would be to drill the holes oversize, and use #8 screws, but only if there is enough material to do so.

The hum/buzz you hear may be the e-unit coil. If it is, there is not much that can be done to silence it. The hum/buzz of e-units is part of the charm of older Lionel locomotives.

The motor can be dis-assembled and the brushes, brushplate, copper commutator and brush springs all cleaned with hardware store naphtha. Reassemble the motor and lightly oil both ends of the armature shaft. Oil the wheel axle bearings, and grease the gears lightly. Clean the wheels and roller pickups with the naphtha also.

The rear truck with the drawbar can be replaced as a unit. Brasseurs has it, part #1130-20. http://www.brasseurelectrictrains.com/service/lionel/720to1600.php

Larry