MG Car Repair: 1973 mg mgb, drain plug

Question
where is engine block drain plug or tap located.

Answer
It is just above the distributor, slightly to the left.

Early cars had a tap, later cars like yours had a plug.  

See part #2 in this diagram:

http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=28976&SortOrder=2  

You could fit a tap to your car.  It is a straight replacement.