an update for future reference...
for key repair, yamaha service staff charges $88 per trip. how many keys they can repair per trip i am not sure. but i am sure not all 88 keys.
changing the whole keyboard assembly costs $190 including labour, trip, GST, part, install... ie, supply and install. this is the option i take. also yamaha provides 1 year warranty for the keyboard.
i also asked if i can buy the part (whole keyboard assembly) from yamaha and DIY install by myself. they said no.
hope this helps.