Do you know what it looks like? It should be a black plastic bulb the size of a fist and should be mounted along the centerline of the boat. I would locate the control box and with the help of the manual which should be available online, identify the wires going to the fluxgate compass and follow the wires. It needs to be away from magnetic and electronic fixtures so look along the centerline in the underside of the cockpit area as a likely place.