X2 on JP's LED suggestion. You might also install a newer-type sealed AGM battery which, at 4.6ah, more than doubles your stock leaker's 1.9ah charge capacity.
This is where a lot of gray area, including "it depends", enters the equation. If you don't use the headlight as a DTRL, then the battery will charge with modest mileage during the day. In this scenario, you could rewire the headlight so that it draws DC power from the tail light circuit. Keep in mind that when the headlight is powered, you will be running a partial-loss electrical system...meaning that the battery will discharge. However, 25w @ 6v equates to ~4.2a current draw, a 4.6ah battery should take roughly an hour to completely discharge, running at total loss, i.e. no charging at all, like with the engine not running. With the engine spinning a more or less steady 5000rpm+, there should be at least 10-15w of DC power going to the battery. I'd guesstimate closer to two hours worth of usable headlight power, with the higher capacity battery & LED tail light.
This is what I consider a half-assed, but potentially workable, solution. The reason being that, IMO, the headlight should remain powered as long as you care to ride...even if that's 12 hours at a shot, without discharging the battery. That said, I've been succesfully running a 12v/35w headlight, as described above, but with a 12v/5.0ah battery since 2006 and have never so much as connected a trickle charger during the winter. Longest ride with the headlight powered has been 45 minutes. Battery voltage was still 12.4 volts at the end of that ride, same reading I've gotten from this battery since day one. I don't use the headlight during daylight hours and rarely ride at night; also, this bike is powered by a Honda Nice 110, which has a more robust, 6 coil, 12v, alternator. Aside from using 6v parts and scaling-back the wattages to better match an old-gen, 6v, alternator, I cannot think of any reason why this wouldn't work the same on an early vintage CT70.
Just FYI, the reason for tapping battery power for the headlight (and leaving the yellow AC lead unused) is twofold. First off, you will get full brightness until battery voltage, under load, drops below about 11 volts. Second, I dislike a headlight that dies when the engine stops.