i'm not sure if a biturbo will work on a garelli ... performance exhausts are custom made for the cylinder and frame of the bike they go on.
but ... there are several other performance exhausts out there. i have a polini custom exhaust on my bianchi. my friend dan has a leo vinci on his carabela. both are minarelli engine mopeds. i know that malaguti makes performance exhausts, too. at least for vespa.
i don't see why you couldn't get a polini or something for the garelli. see if anyone else can tell you where to get one. i think some people here know more about that.