Four or five cell pack? Is the battery freshly charged and KNOWN to be a good battery?

How did you load the battery with your meter? Do you have a volt meter or a loaded volt meter specifically designed to evaluate a batteries charge?

Simply because your battery reads voltage does NOT mean its fully charged or providing current under load... Check the battery.

If you are looking for a sparkplug make sure you obtain an exact plug replacement or cross over... I don't recommend useing a non-resistor plug. I doubt you've run the engine long enough to harm the plug. It maybe dirty and or partially fouled from the weak spark at rpm, improper needle setting or timing related problems...

Where are your needles at now?

If the engine starts easily and such its unlikely the timing ring has moved.

Most likely the problem is the battery, then the ignition module based on your information.