Keyboard contact pad restoration

[Tugdual]

I just purchased this device on flea market

This Philips SBC1705 seems to be a Casio brand named with no programming feature but it is slim, has Bin/Hex/Dec and the design is somehow satisfactory. Not exactly sure how old it is.

Had to change batteries that (of course) had been left and leaked but the second issue is that the keyboard rubber contact pad dried and is torn up in some places. As a result the keyboard is a bit stiff and some keys require a strong push. Any idea how this could be restored?