ORIGINAL: dirtybird
A two cycle gasoline single cylinder engine will vibrate. There is no way to get around it. Your installation may have a resonate spot at low rpm that exaggerates the problem. Make sure the engine is secure and avoid that RPM if possible. I doubt if a small spot of epoxy on the spinner has anything to do with it.
This statement is a fact, and the fact that you dont know how to balance a prop, explains your vibration. If the engine is performing to your expectations, chances are you have a good working sparkplug. Since you are going to change the plug, make sure its gapped to .020 and BALANCE that prop.