Oil leak from scooter?

you answered your own question, the bolt you say is missing is more than likely one of two that holds your exhaust downpipe to the barrel (cylinder) and because your bike is a two stroke its burning oil which will end up black because of the combustion process and leaking from where the bolt is missing. If you stop using the bike and replace the bolt you might be lucky and get away without replacing the exhaust gasket. By the way 2 strokes do use transmission oil but this isn't used anywhere near the piston/ combustion area and so should always stay relativity clean .