If you don't want to make a diode step down, you can buy them already made. Wrong Way RC sells them along with Align Helicopters. Wrong Way sells them as a voltage step down for ignitions where Align sells them as a voltage step down for tail rotor servos that are not rated for higher voltages. Even JR use to sell these but I'm not sure whether they are still available or not. Much simpler than a regulator for this sort of application and almost foolproof reliable. Couldn't agree more with Dirtybird this is the way to go and this is a plug and play solution.
Also, with all the speculation regarding voltage, why doesn't the OP just call Ralph and see what that older ignition is rated at. Why rely on keyboard experts when the original source of the ignition is still available to answer any questions? That is clearly one of his early conversions with mechanical advance.