Try this. Remove the spark plug, and place a large rag in front of the spark plug hole. Ground the spark plug wire. Turn the engine over, try to start the engine. I'm guessing that the cylinder is filled with a gas oil mixture. You will see gas fill the rag. You ground the plug so there is no chance of a spark, as you are going to get a lot of fumes. After no more gas comes flying out, install the plug and change the oil. Then try starting the engine.