Simplest on-board glow is a battery and a resistor wired to the plug. You don't need full voltage of a single NiCd to give increased idle reliability.

Measure the resistance of your glow plug. You want a resistor with 1/2 to 3/4 of that value.

Put a Phono type jack between the resistor and the glow plug such that you can boost the glow power for starting by simply plugging in a 1.2 v source. Add a switch between that and the glow plug to turn off the on-board glow.. and allow charging with the boost jack. (don't have the boost jack cut the on-board battery out... vibration could then cause RF noise.)