Your first two sentences is why I asked what engine it was. Depending on the engine there may already be an RC Exl that's "off the shelf" ready to go for the engine and plug type. On most single cylinder ignition systems an 1,100 mAh 4.8v battery will provide dependable power to the ignition for several hours of continuous running.

Normally an ignition battery that gets weak will cause the engine to run a little rough, with problems on the throttle up for high rpm.