The only thing I've heard of even remotely like this is an exhaust cutout... which if installed after
an expansion chamber on a two stroke - but before the muffler on either a 2 or a four stroke - allowing you to open a bypass to achieve full flow (and noise) or close it for that "stealthy" approach.
If I thought there'd be enough of a gain on these lil bikes - I'd do it meself
Having something after
the muffler that simply closes I can't see doing any good for anything other than a back-up kill switch o.O