There's a lot of owners of Kawasaki-powered ZTR's who would like a few more HP, but attempting to fabricate a "performance" exhaust is probably not the best idea to get there.
Kawasaki engines often are manufactured so that one basic unit can be offered in several HP ratings. The engine size (displacement), as well as 99% of the part numbers, remains the same, but some relatively minor items, such as the carb shaft are different. Thus the same series engine can have a 19 HP rating with one carb shaft, a 21 HP rating with another carb shaft, and so on. A HP range of 4-6 can be expected, normally.
Since the carb shafts are interchangable, it could be a simple matter, for example, to purchase a 23 HP carb shaft for an original 21 HP engine to gain more power.
More power means a decrease in fuel efficiency, of course. Another downside is the possible need for an oil cooler. The upper end of a series of engines will often come with a cooler, so it is best to duplicate this practice when swapping carb shafts.
Maybe the engine in your ZTR would fit into the upgradable catagory.
Here's a discussion from another forum wich will help explain the procedures:
http://www.lawnsite.com/showthread.php?t=317558