You should be able to use a '56 Ford or Mercury Y-block 12-volt starter and generator, as well as horn relay and horns.
12v instrument bulbs and headlamps can still be purchased new. If old wiring is good, it does not need to be swapped but you will need to add ballast resistor and 12-volt coil for the ignition. You'll need a 12-volt heater blower motor, probably from a '56 (but not a '57 or later as they rotate backwards). Clock and radio will need to be converted to 12-volts. Most instrument service providers can fix for you as well as restore them cosmetically. Some minor wiring modification is required for the start circuit relating to the ballast resistor. Use 1956 or later shop manual for wiring diagram.
EVERY lamp must be changed to 12-volt. Don't forget some of those well hidden bulbs under the dash like your PRNDL indicator lamp and your glove-box interior lamp.
If Merc still used the old-style varnished cambric jacketed wiring in '54, you should inspect it all well to make sure that old insulation is still sound. If not, the repro parts suppliers sell new thermoplastic jacketed wiring harness but make sure you are sitting down when you see the price. I believe you can still buy the OEM style Sparton horns from repro parts suppliers. The power seat motor, and power window motors should be okay as is, but you may need to swap out the power window relays in the doors as I am not sure if the coil will handle 12-volts very long. The contacts are double what you need but since they come in a unit, you need to buy the whole thing. Voltage regulator for your battery can be a standard 12-volt 40amp unit from a '56 or later Ford.