Cheapest way will probably be another block. Either option requires a full stripdown anyway really, at which point you might as well do any other work that arises (new rings? Mains?). From that photo it looks to be a 3mb block (no X bracing on front of the drain plug) which would be odd in a 1981 motor I think (unless it was one of the last 3 bearing motors, but I'd have thought they were all 5 by then?)
Looks a very nice unmolested motor though, and we'll worth spending the time to get it spot on!