If all one is able, or willing to do is clean the hole where the drain petcock is, one may as well not do anything at all. There is a huge area in the water jacket which will be packed with crud that you cannot access without tearing the engine down.
You may think that cleaning that small area to allow some water to come out is a good thing and therefore makes you feel like you have done something. I suggest that you instead would be giving yourself a false sense of security.
Since an engine which is loaded with crud likely has not been worked on for a very long time, you can just run it as is (as long as you don't have an overheating issue from a completely clogged water Jacket) until you decide to tear it down for a proper rebuild. Then you can actually clean that water jacket out.
Carl
