If it's just leaky valves, it doesn't cost anything to fix. You can normally tell just by looking at the valve seats: if they are black, they are probably not sealing. If you have a good contact mark the whole way around, they are sealing. Unless they are badly burned, you only have to lap them to get them sealing properly. Then check the tappet clearance, of course: lapping the valve removes metal, and that reduces the tappet clearance.
I always recommend doing the oil test before taking the head off, because once it's off, you can't tell if the rings are any good without putting it back on again.
