get a 2t 50cc and whack a 70 kit on. the law will never notice. also there are some aftermarket exhausts available from leo vince, gianelli, etc that are higher performing but look stock.
'07 Vectra SRi
'11 Honda Hornet
'99 Aprilia RS50
'80 Honda H100A
'82 Puch Maxi S + '70 Puch Maxi N