coolant not flowing through the radiator usually results in overheating. low compression in an engine can only be remedied by rebuilding the head or the block or both. blow by can also only be remedied by rebuilding the block or installing a oil catch can will help reduce it but it does not fix it. check if your oil is actually milky if it is it will look like chocolate milk. because if a mechanic noticed this and let you drive away i wouldnt go back/. its kinda like cancer. catch it early in the milky stage and fix it chances of survival are good. leave it alone and the engine will blow up.
and by rebuild the ignition system, what parts were changed?
i would say make sure your head gasket is really ok first. then maybe change the fuel filter, clean your air filter, and make sure your tires are inflated properly. that can cause your mileage to drop.