Keep in mind if you're searching for a non-fender replacement that all Fender 2 button footswitches are not the same and all will not interchange. Also, few non-fender designs function with many Fender amps. Fender uses a unique design in many of their footswitches that is based on voltage changes (accomplished w/ diodes in the fs) rather than simple latching circuits. They did this to allow the use of a common guitar/instrument cable as a replacement rather than requiring a stereo type cable.
As for your specific Fender FS, I don't know, but often the FS schematic is included on the Amp schematic downloadable on Fender's website. But really, if the above suggestions don't work, you're better off (less grief) buying a new Fender replacement with the correct part #.