1/2 NPT (national pipe thread)
Remove the fitting in the manifold and go to the hardware store. You have several options, match up the threads in the plumbing section to be sure. Then look for pipe plugs, they come with different ways to turn them and different materials. You should find ones with a recessed allen wrench (brass or steel), square head iron (cheap and ugly), you might find a hex-head in brass. It usually will be a tapered thread so it will just need a little thread sealer and snug it up. The tap for 1/2 NPT should be cheap, check to see if the manifold opening is the right size for the 1/2" tap. A 12 pt socket will work for a tap handle, tape it to the socket so it doesn't keep falling out. Grease the tap and most of the cuttings will come out with the tap.
Have fun....!