The repair guy at our local official Yamaha repair center quoted Yamaha's price for a new main board at over $1,000. He said that even if he installed it, he could not guarantee that the kbd would work, since he feels it has other issues (he can't prove it, but he thinks it may have taken a hit from a power surge or lightning....
What you and your church need now is a SECOND OPINION on the troubleshoot and estimate.
In this old electronics troubleshooter's experience, what he says just doesn't happen very often at all. Statistically absurd.
I suspect a Power Supply issue and that can be troubleshot and repaired.
Take the unit somewhere else.
--Mac