Do you mean the screw that fix the horn to the output axle is stripped out in the head? Or is it on the horn itself where you connect the link to the throttle?

If it is the head that is stripped you can try to "drill off" the head, take off the horn and use a plier to screw the rest of the screw out of the output axle.

Anyway, replacing a horn itself is easy, off with the old and on with a new one The servo is too good to throw in the bin just because of that little screw