Generally old bikers discard the batteries or the batteries are dead since pakistani motor cycles work perfectly even without a battery.....
in this situation the magneto coils act as dynamo to head light as with higher speeds the volt at times exceeds 6v and (I) fuzes
Solutions(DOs):
new battery if affordable
try using a 9v but higher wattage then original... bulb
a small resistance in series to bulb
get regulator repaired or replaced
other solutions(DONTs)
wind proof laaltan
torch
drive under street lights
use night vision googles
etc,etc.....