Your seat can be done by Sargent seats or you can buy a new one from Corbin for $230.00.
http://www.sargentcycle.com/default2.html
http://www.corbin.com/
The 84 ZX900 (A1) is a Ninja 900
Some model differences, but nothing too major. 85 (A2)and 86 (A3) US
In Europe, Japan, UK etc the bike is known as the Gpz900r and had run from 1984 - 1996 (A1-A8)
The main difference would be in the later years they went to a non anti dive unit and 17" rims instead our 16" rim in the front and 18" in the back.
Parts are available through Kawasaki and when not available or not a cost effective soloution through salvage yards and of course, don't forget ebay!
With the bike getting older I have considered getting a whole parts bike to keep it running right. Initial investment is more, but cheaper than going to the dealer all the time!