The best advice I can give you is to use a z22 engine from another 81 or 82 720. Anything else and its going to be too much work, its already a tight fit getting the engine and tranny in there at once. The wiring is also a b**** to take out and put back in and thats with the original s***, i cant imagine what kind of nightmare youd have replacing the entire undercarrage engine and electrical without extensive prior experience.
That said, even if you did that the Datsun body is notorous for rust and doesnt take damage really well in accidents so rebuilding anything with it would be a waste of time.
My dad and I have gone through 3 datsuns over the years. The first 81 he bought back in 1982, it ran all the way until about 1994 when we bought an '82 as a parts truck and used it to replace much of the engine, transmission, and rusted body parts. Then in about '99 we used the old block from the parts truck and rebuilt the engine entirely and put it in the 81. Almost everything from the 81's and 82's is interchangable, little to nothing from 80 and 83-86 is interchangable (to my knowledge). Anyway, that newly rebuilt '81 got totaled by some ***hole with no insurance after only 10,000 miles, so we picked up a freebe '82 that woulda gone to the junkyard otherwise and put the engine in there.
Lots of those old datsuns are pretty decrepid just sitting in peoples driveways until they finally submit to laziness and send them to the junkyard. Both the 82's we just saw them sitting in our neighborhood and made the owner an offer, the first one was 500$ second one was free - so I think youre best bet is definetly to find another parts truck that still has the working parts you need.