You simply need to use a higher-capacity battery (one with more mAh), or multiple glow drivers.

On a twin it would be easy. Use a 2-cell battery, connect the positive to the tip of one plug and the negative to the other. to run the plugs in series.

However, as you're probably well aware, you can't do that on a 5-cylinder. All the plugs need to be wired in parallel, unless the cylinders are electrically isolated from each other. Since the cylinders, crankcase, pistons, connecting rods, and crankshaft are all presumably metal, you can't connect them in series.