The Copperhead ignition was just a rebranded RCexl ignition. Some of those early ignitions preferred to run at the lower voltage so I would question whether the use of a A123 battery connected straight would be the best thing for the ignition. If it were mine, I'd use the A123 battery but would use a single voltage dropping diode in series with the + battery lead. This was common practice a few years back. Wrong Way RC still sells these diode / cable assemblies or you can make your own. Very low cost and any worries of over voltage damage go away.