Personally, if your local authorized repair guy won't diagnose the problem accurately (or he would KNOW that a board swap would work or not), I would call Yamaha and ask to ship it back to the MAIN repair center, and let guys who do NOTHING but work on Yamaha products sort out the problem.

You wouldn't let a surgeon operate on you until you had a definitive diagnosis, and I wouldn't let this doofus work on your Motif until you KNEW what the problem is...