You do not need to tap the hole. Just throw something in there. Don't matter what it is - a chunk of aluminum, steel, brass, wood, silly putty, whatever. If its not a 100% seal it will still be plenty enough to restrict oiling to a non-critical amount. I never do it - but you certainly can do so if you wish. The thing you slip in there will not go anywhere - can't get by the head bolt down low and can't get past the rocker bolt up high.
If you are convinced that you have too much oil you can also purchase restrictor pushrods with a smaller diameter hole in them - like maybe .015 in diameter.