Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
That (the Korg OASYS 88) may have an AOpen motherboard, like this one:
which even if it were a good replacement for the one you have, is no longer available for sale (in that listing anyway.)
Why not look on your Korg's main board, and find out what (presumably AOpen) board it is? then we could either/both search for that specific board. As a 'Korg board' it may not be findable, but as an AOpen board it could still be found.
Here is another listing for that board:
OK. Let me know what you think, at your convenience. Thanks.