Small Engines (Lawn Mowers, etc.): white smoke from briggs 17.5hp, blown head gasket, mosqitoes

Question
I recently purchased a used mower from an individual.  I replaced battery, and noticed that the engine oil was overfilled.  After draining the oil i noticed that it had gasoline mixed.  I filled to the appropriate level, checked the air filter and tried to fire it up.  it started with the white smoke, i let it run for about 3- 4 minutes and decided that i better shut it down.  what is the next step for me, to try and get this thing running correctly?

Answer
2 or 3 possibilities on this one. I will go from easy to hard.
1. Check if carb is still leaking fuel into crankcase, if so clean carb.(black smoke)
2. Blown head gasket. this is very common cause for smoking on OHV engines. (blue or white smoke)
3. You got ripped off.(fogging for mosqitoes)
If the answer is 3 have a garage sale.
Good Luck