I've been think about what bn120 said, why do we have spark type systems and I got the answer.
With a spark ignition the plug fires at whatever degrees before top dead center its supposed to but with a glow plug it fires when it wants to. I think thats why you lost rpm with glow and a battery, its firing at less degrees BTDC and even less without the battery because the plug cools off and takes longer to ignite the gas.
His idea about the four stroke plug should help but without any way to control when it fires you may never get to same RPM as a spark plug.
h82 crash is right, I had a GMS .47 start with out a battery, I was flipping it prior to attaching the glow driver to clear it of extra fuel and it started. Woke me right up, glad I was using a chicken stick.