Hi, You don't say whether your Sunbeam has a dynamo or alternator (which I doubt). Either way, changing to 12volt is generally possible. A dynamo will need rewinding, but generally the wires of an alternator can be fed to a new regulator/rectifying unit designed to output 12 volts. Dynamo systems would also need a new 12volt cutout/regulator.
Of course all voltage sensitive units like ignition coil and bulbs will need to be changed to 12volt types. Switches etc. should be OK and possibly the horn, also the ammeter if fitted.
It seems that most people think changing to 12volts will magically double the power allowing much more powerful bulbs etc. to be used. Unfortunately that is not usually the case unless a larger dynamo or alternator is fitted.
If your concern is lighting, a much cheaper alternative is to fit light emitting diodes (LEDs) in place of bulbs wherever possible, which should enable you to uprate the headlamp bulb by several Watts.
My Speed Twin has been treated this way and with a 10Watt larger Halogen headlamp bulb which fits the original light unit and much increases the light output.
If you are interested in investigating this option all the LEDs and Halogen bulbs with standard caps are availabl from Paul Goff.
www.norbsa02.freeuk.com