Not available for order at Yamaha 24x7.
Found
this parts vendor though. Unknown whether they just order from Yamaha and resell the part. Might be worth emailing them and asking whether ordering would be worthwhile.
Found
this UK parts vendor. £536.04 is awfully steep, though.
Depending on what's fried, your existing board might be repairable by an expert. Near where I live, there's a mobile phone repair shop that can replace resistors, caps, diodes, whatever simple components are fried. They work on game consoles, laptops, and pretty much anything with a PCB. Maybe you've got such an electronics wizard in your area? As long as it's not a processor or EEPROM or similar more proprietary component that's fried, your board might be repairable.