I'd be inclined to go into local locksmiths and see what they can do for you. The Yamaha keys (from memory) were poor quality and mine had a tendency to twist (not helped by the lock mechanism).
You could aslo try someone like Fowlers here in the UK : >
https://www.fowlersparts.co.uk/brows...acturer/yamaha
Not sure if they sell the keys, but might be worth an email/phone call