If the guts are inside all you need is 12V bulbs. Sorry, I don't know the bulb number but somebody will chime in shortly with it.
Your original headlight switch will work fine on 12V. Just run a wire from it out to the headlights. The lights ground through the mounting, there's a copper tab inside that goes from the reflector to the bucket,then it grounds to the mouning bolt, the mounting arm, the hood, dash/dog legs/radiator brackets to the engine/torque tube and back to the ground cable attached to the battery.
Here's a picture from TM's website (Site Sponsor link at the bottom of the page) showing the headlight assembly.http://www.tmtractor.com/new/el/4916fp.htm
Here's another option to consider, not original but better light and with 12V's you should be fine on amperage.http://www.farmallcub.com/phpBB2/viewtopic.php?f=140&t=70780
I think the comparable 12V bulb is a 4411.