RE: OS120AX quit & wouldn't restart
Even though a glow plug will glow when you apply a battery to it, it can still be bad. The platinum plating on it can be damaged. It should still fire when the battery pack's applied, though.
If you have good compression, a good plug, and good fuel, the engine WILL fire. If it won't fire, then the engine is either too lean so that there's not enough fuel to fire, or there's too much fuel (flooded) and the excess fuel shields the plug from causing the fuel/air mixture to fire. Not much to actually go wrong.
If you're having difficulties, close the needle valve, remove the glow plug, put a rag over the glow plug hole, and spin the engine over for several seconds (or vigorously flip it) to work out any excess liquid fuel that may be in the engine. Replace the plug; use a new one if you have it. Keep the needle closed, but energize the plug and try to start the engine. After a bit, it should fire and run for a few seconds, burning the remaining fuel that's present. Keep doing this until the engine won't fire. Then open the needle valve to the normal starting position, prime and start. Be careful to not overprime the engine.
Your engine should start easily. Most starting difficulties are due to something in the technique or settings, and not anything actually wrong with the engine. Again, if you have good compression, good fuel, and a good glow plug, the engine WILL fire.