Since I suspect that the keyboards were originally done by silkscreening conductive ink onto mylar, it should be fairly easy, albeit time consuming, to make a new mylar. My guess would be to use something like a Houston Instrument Pen Plotter that would also take a vinyl cutting knife, cut out the mylar, make up a screen, and just silkscreen the traces onto the ready to insert mylar. Or, if the old mylar is not broken, just reuse the old one. I don't know what kind of prep needs to be done in order for the ink to adhere to the mylar, but this (and anything else that comes up) is something that has been done for a long time. Another thought would be to go to someone who already produces this type of switch and find out what it would cost to make up a couple. It might be possible (depending on the size of the company) to get it done gratis, ie as a sample.