A. NO. The 84+ rear head pipes won't fit on the '81 to '83 mono-shock bikes. The '84 up pipes are too short to clear the swing arm and turn too abruptly to fit. Likewise those early models will not fit the later years without interfering with the tire.
Additionally, the front head pipes mount slightly different so those won't
interchange either, in spite of looking similar.
Comments: You can cut the early pipe and use an extension to replace the rusted parts. But stay with the stock diameter of the pipe because it fits into the muffler system. If you can get the rear header for the later model you could have someone cut it and weld the relevant piece back on your old pipe...but that could cost more than going to an exhaust shop and having them weld a repair piece onto your stock pipe. You can also check ebay for a replacement from the same year.