What is wrong with my car and how can I fix it myself?

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2003 Buick Regal Symptoms include: -started studdering in idle at long lights last week -2 days ago, lost get up and go at acceleration, studdered, and not able to get above 30mph. -was told its misfiring, changed spark plugs. Timing is off slightly -overheated, wouldn't turn over -smoke coming from where oil goes into, ONLY! - found mud in radiator, flushed it out. Haven't blown it out tho. -idle and drives rough, studders, stalls when slowing to make turn or at a light -recently had a oil and oil filter change, and new fuel pump put on.