I don't know about Mercury outboards but they should basically be the same. You need two things on it: the generator, which consists of one or two coil assemblies; and the regulator/rectifier, which is basically a box that plugs in between the generator and your boat. There might be related wiring and brackets needed too.
You should be able to go to your Mercury dealer's parts department and they can sort it out for you. Or, you might be able to find the part numbers online like here