Tuners usually make expansion chamber them selves and according to the power characteristics they are looking for just figure out where you need the power in the mid, high, low etc (there is a whole topic on how to make expansion chamber in the 2 stroke tuning book by Graham Bell, good read let me know if you're interested I'll send you the pdf file) make the design on paper and get them cut and welded from exhaust guys that are everywhere.
as far as cabon deposit is concerned they are lots of ways to get rid of them from hot tanking, acid bathing and even torching it will remove it
why would yamaha make an expansion chamber for a mope, they arent for racing but if you look at other yamaha 2 stroke bike you'll see expansion chamber on them