Furuno USA doesn't sell individual solder-in components except for a few variable resistors used on front panels and chassis jack connectors. The diode would have to be purchased from an electronic supplier if you know its value or the entire board would need to be replaced. Furuno USA isn’t given many board schematics nor do we typically do much repair beyond the board level. You might try working with some of the local dealers who might keep used equipment and sometimes does component level bench work.