I think that the only upgrade would be Thunderbolt ignition which would require the Lightning Energizer, switchbox and coil; (ignore the color coding) if you're lucky the splines on the Ignition driver will match.
you could also replace the plate rectifier with the diode type.
Not many parts will change as Mercury made major changes in 1964, in their effort to Silence the Beasts. Though things like Carbs and Ignition are likely to be transferable.
