If you are looking for the correct stock system, you may be limited to the 1977-1980 A model. Partzilla has a pretty reliable crossover chart. If you want an aftermarket system, there are more possibilities. I believe all the 900s and 1000s through 1980 use the same header and mid pipe. Generally, the only difference is in the muffler hanger, and that can often be solved pretty easily.
There are new systems available as well.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough
