[raymo]

Go to your local hardware store and get a set of "easyouts", drill through the screw with a small drill [around 1.5 or 2mm] and screw the smallest
of the "easyouts" anticlockwise into the hole using a small spanner or shifter on the square end of the "easyout, which will undo the stripped screw. Have the objective end of the scope higher than the focuser end when drilling,so swarf from the drilling stays away from the lens.
Ensure that you drill as near centre of the screw as possible, so you don't damage the female thread in the focuser. Two minute job with the right gear.
raymo