Because 2 Stroke engines do not have any valves as such, they require a very specific exhaust shape in order to utilise scavenge pulses travelling at the speed of sound to return unburnt fuel/air mixture to the cylinder before the piston reaches the top of it's compression stroke. This means that it is only really possible to have peak operating conditions where engine revs, fuel mixture and scavenge pulses are all working in harmony over a fairly narrow band. The rev range in which this harmonic balance takes place is decided largely by the length and shape of the expansion chamber of your exhaust.
By measuring the exhaust and transfer ports of your cylinder and taking into account your preffered powerband range (some people like a longer spread of slightly less power for easy riding whereas others prefer maximium power but with the trade-off of a very narrow power band high up the rev range) an exhaust can be designed using proven formulae and Computer Aided Design software to produce a bespoke system for you.
These systems can be made out of stainless steel for longevity on street and show bikes. For racing I would advise mild steel (unless you have a massive budget) parlty due to the likelyhood of crash damage but also because the lower cost means it's feasible to have pipes with different characteristics for different tracks.