You have to compensate for that, i.e. make sure to establish good conductive properties of that failed line(s).
In short, get a pen with conductive ink - many electronic stores have it. Then make two or three thick passes until you get a visible, dense line, again.
Or maybe just get a spare keyboard (without keys and metal casing, to reduce cost on freight). You need small PCB - printed circuit board (stuff in upper right corner which connects to A500 motherboard) and plastic membrane/foil which can be detached/attached from PCB and rolled for smaller package format.