I would be possible to convert to full DC and would require a small mod to the stator to disconnect the lighting earth wire and connect it to an additional lighting wire. (PX/T5 stator is very similar to Lambretta). BUT, your stator won't be able to produce any more than about 70 watts which is not enough to power 2 x 55 watt headlamp bulbs. Using a decent capacity battery (7 Aph) would give enough battery power to light to headlamp on full beam but only for as long as it took the battery to discharge (probably about 30 minutes or so) as the limited stator output is not enough to charge the battery as quickly as it discharges...or to use Monty's analogy, the hole in the bucket is bigger than the hole that it's filled by
That could work if you only do short trips that require little use of full beam and you are able to re-charge the battery from the mains on a regular basis but otherwise not.
You really need to rethink your headlight arrangement, 2x 35 watts would work in theory but you would only have the power needed when the engine was revving fairly fast (maybe around 4-4.5k or more) below that the main is going to be progressively dim if you used AC for the headlight but with a DC conversion the power would last a bit longer.
It might be an idea to look at the twin LED Spots that Knowledge has fitted to his GP in place of the headlamp, they are apparently very bright and require a fraction of the power of filament lamps, these are similar
http://www.ebay.co.uk/itm/125W-Motorcyc ... 3a95d0f5b2
