I tried changing the oil in my son's CRF 80 and now it won't turn over.?

I know that it is a 4 stroke so oil goes directly into the engine instead of the gas. I changed the gearbox oil and filled the engine up but it kept spurting out. After I got half a quart through that tiny hole in the engine I installed the spark plug but it wouldn't start. It felt like the engine was locked up but when I took the spark plug back out it kicked over fine but oil shot out of the engine. Anyway, every time I fill the engine up with oil it won't kick over. How do I keep the cylinder full if it won't even start? Should I get a new spark plug? The gear box oil was fine after I changed it and I have only had this problem since I changed the oil in the engine.