I would narrow it down to see if your compression loss is rings or valves. Pull out all the plugs, run a compression test, then quirt a little oil into each hole. If that brings the compression up to near normal, you have excessive ring blow by. I it does not, you probably have a burned valve. Lapping only does good after you grind the valve and seats to put the final touch on it. If you just lap the valve, you will just make the seats wider and they are probably too wide already. While you have the head down, you should measure the guides. Replacement or knurling may be in order. If the engine has been set up for a while and shows excessive blow by, you may have stuck rings. Squirting something like Marvel Mystery Oil in each hole and letting it set about a week might help that.
