Ha, funny. That pic of a Foxi you posted happens to be a bike we reconditioned some years back and it also just happens to be here in the shop getting a kit put on it along with a G3 exhaust adapted to the left hand side. The problem with the Foxi's (and there is not a lot to complain about) is they are a 25 mph bike and their small stock pipe and bracket is located on the right hand side of the moped where the majority of Sachs powered bikes have a left side exhaust. If you want to go over 25 mph that pipe has to upgraded to something that can actually breath.
Email me direct at: firstname.lastname@example.org and I will email you some pictures of the seat mount. Dont know how much it will help though, there is a lot going on under there and it may be a bit difficult to get a good perspective as to how everything connects.