There must be a bung in the hole otherwise there would be leak, is that a screw I can see in the hole? The early ZD30 doesn't pay much attention to the MAP, I had had mine (2000 model) disconnected for a while back in the old days when I was looking at cures for limp just to see what happened and it didn't care, so they could have just dropped it off, are you sure there are no "spare" plugs at the front somewhere.