Definately put one in that works. Dont bother trying to block it off. I did when I first put my new engine in and it was a pain in the ass. You could see the capour and stuff coming out of the block pipe even though I had it facing the ground + you could smell it. Also the line that goes into the aircleaner. Oil was pumping up it. In small doses but it didnt look clean inside the air filter element.

It just wasnt worth the hassle. I noticed no difference in performance or idle ect when I put it back in. There just didnt seem to be any other effective way to ventilate the engine properly.