It's a 1N004 diode that you use. Each diode drops the voltage by .7 volt. With a 6.6 volt pack I myself would use 2 of them in series because that pack is a good bit over 6.6 volts when charged, and even after being used for a while. (The ignition only draws 300 ma, (.3 amp). Your ignition will thank you for the kind treatment. These ignitions have been shown to give good spark with only a 1 cell Lipoly at 3.7 volts.
Look online for the diodes. Digikey, Mouser, etc. will have them. .25 cents each?? Don't know 'cause it's been a long time, but they're cheap.
AV8TOR