There's probably lots of things that could be wrong. I know I oil fouled the spark plug once or twice and it wouldn't clear itself, it would cough splutter and carry on, then you could wind it with the starter with only the occasional pop. Pulling the plug, washing out the oil in the plug, re-installing and restarting the motor upright letting it blow out the excess oil before returning it to inverted was the only cure.

A spark plug won't clear itself like a glowplug if the motor gets an accidental gutt full of fuel/oil.

Or it could be the wire from the hall sensor that was just about broken, and the stress of putting in a new plane was the just enough to break the last strand?

Try to work through one thing at a time...