This is not a DIY project for a green newbie.
You may find a replacement part at
http://syntaur.com/yamaha_motif.html
or at
http://www.fullcompass.com/brand/YAM_Yamaha/Keyboard-Parts.html
but removing and replacing a key is a massive undertaking for someone who has no idea what he's doing.
A professional service tech can do it for you for a couple of hundred dollars.
If I were you I would not try and buff out the scratch or fill and polish it in any way while it's still installed in the keyboard as the dust and crud it would create would probably do serious damage to the Motif's operation.
If it's a minor cosmetic blemish and does not affect playability in any way my advice is to own up to the accident, buy your brother a bottle of scotch and a new effects pedal as an amends gift and bury the hatchet.
These are working instruments. In real life gigging musicians ding and scratch their instruments every day and pretty much live with it unless it becomes totally dysfunctional.
Otherwise, if you own a $2000 keyboard and you can't live with a slight imperfection then you just have to bear down and eat the $200 to $300 a service tech will cost you.
This is the best advice I can give you on this.
Gary
