you are correct in thinking that it could be the crank. how ever the cam should go well before the crank. i have rebuilt a couple of these that have been ran low and or out of oil and it has been the same on all. i just had to rebuild the top end. one of the motors have 15k after the top end rebuild and its still going strong. you can buy a master rebuild kit for $500 plus what it would cost to machine the heads. even if you take it to a mechanic it should cost less then a motor swap.