Just came back from the Goodyear shop to have my codes read. They came up with two of them, P0301 & P0308, I know they have to do with misfiring. The tech in the shop told me my heads were not doing too well and there wasn't enough compression which would be the reason I shake and rumble while at a stop light and while accelerating. My question is how much would it be to fix this problem and how difficult would it be to go to the yard and pick up some parts and replace them? I'm fairly handy around my engine and I have a lot of help from mechanic friends I know. Any help would be greatly appreciated.