These keyboards suffer from the rubber strips which act as the springs under the keys getting soft and the strips have to be replaced. Once you disassemble the keyboard you will find these either blue or green strips that look almost like 2 rows of bubble wrap under the keys. Yamaha have stock of them and they are not very expensive. There are multiple strips which handle about 10 of the keys per strip. It's best to replace all the strips at once for obvious reasons.

You should be able to replace the cable with a standard USB cable similar to the ones used for portable Hard disk Drives. ie slightly bigger than the ones used for android phones. It will have a standard USB connector on the one end and a smaller connector on the other. You then buy a cigarette lighter 12 V USB charger from any of the cheap 2 dollar or gas stations and off you go. You are bound to have a cable lying around somewhere!