Several things to consider here.
Manufactures produce parts for popular high volume models, they are after all in business to make a profit. The call for expansion chambers for 90cc Kawasaki's was likely never very great. Nothing is impossible but the chances of finding a pipe for an old 90 is very unlikely. However all is not lost, there are resources that contain the formulas to construct a 2 stroke expansion chamber. Along with the chamber you will want to consider porting, compression, rotary valve and carburetor changes. In the end, unless racing in a specific class you are probably wasting your time. Given a fixed displacement there are only two ways to increase the output of an internal combustion engine, you can increase BMEP or you can increase RPM. At any given time in the history of internal combustion engines there is a practical limit to BEMP, that leaves RPM. You can certainly increase the number of combustion events per minute but that will negatively affect low rpm performance. Also every hp added will shorten the life of the engine. At modest increases this will be negligible, but at significant levels engine rebuilds will be much more frequent. Truly there is no free lunch. A high performance small bore engine, and there have been plenty produced over the years, will be peaky and require constant tuning. The higher the output the narrower the tolerance for jetting and such.