Not sure from your post why you want an original OEM under-seat amp from '84 or its replacement. When I switched out my OEM in-dash AMFM-cassette player in my '86na for aftermarket (Pioneer) FM/CD head unit ('86 had no under-seat amp), I was amazed at the better quality of sound, even without replacing the OEM speakers (original). I just got better sound all around and it floored me that I got all that from just a new head unit.
Car audio had progressed so much in those few years ('86 to '89 or '90) that restoring the '86 OEM stuff would have been a shame because of what I would have missed out on. I bought my Pioneer head unit through Crutchfield. They have guides and can tell you by 1-800 phone every available head unit that will fit your '84. The rewiring is not a big deal even if you have little experience. Unless, of course, you are fanatical about reproducing an '84 physically as if right off the showroom floor in '84, you will be missing out on a lot of audio progress. IMHO. Good luck, tho.
P.S. You can also (hugely) improve on those '84 OEM Nissan speakers for a few bucks more. Nissan Z31 OEM speakers suck by comparison.
Check it out. Crutchfield is a good place to start looking.
Be sure to visit the Crutchfield web site before calling them. Everything you need to know is displayed on the site. Good people to deal with. The good reps on the phone really know auto sound.