car had 160k when we bought it. owner said original motor was run out of oil so they put a used motor in with approx. same mileage. uses a qt of oil every 150 miles or so but gets 30 mpg (stick). on start up it shakes so hard the front bumper and fenders rattle sometimes. gets a little better after warm up but never really smoothes out. i've adjusted the valves twice and makes no difference. anything in particular i should look for? is it possible that using that much oil it sat for a long time and maybe the rings rusted up? could the cam have gone flat?

thanks in advance