the Cady pipe is alot different than the regular motobecane pipes. the cady has 2 studs that hold the exhaust pipe on, while the 40, 50 model have a threaded exhaust port that the pipe screws into.
the pipe that i have on my moped is a modded pocketbike exhaust that i purchased on ebay, cut up, and welded to fit the ped.
a performance exhaust will not hurt the motor if it is the right pipe for the right engine. and lso you will have to rejet your engine to not risk a lean air/fuel mixture. my Cady has a massive misfire that kinda richens the mixture to keep it running good. (backfires like crazy)
attached is an early version of the complete pipe i built.