Engine Shake and Loss of Power

What is your question?

Van has about 190,000.00 miles. Engine shows signs of oil leak/build up around head gaskets and undercarriage around oil pan and under engine. Check engine light came on and within a few days the engine lost power and would shakes strongly. Had mechanic run tests and got the following codes: 622/gen file no switching; 442/evap system small leak; 302/cylinder misfire; 455/evap system large leak. Any ideas on root of problem or cost to fix?