K+K Marine or Al Grovers are both certified Yamaha shops on the island. IMO you are better off dealing with them than a private mechanic. What seems to be the problem anyway?
There is a great outboard/marine engines place on Montauk hwy in Lindenhurst on the south side
I don't know if they r specific to Yamaha but they fixed my stuff years ago and the guy who works there is into fish!!