Except for the Suzuki gel seat, any seat you awap out for will have to be modified a bit.  A lot of people have had luck with Sportster seats.  

You can switch out for a Supertrapp, Jardine, or a Sportster (or similar) muffler.  There's a few other exhaust options available depending on how much you want to spend or how exotic you want to get, but those seem to be the most common.

The top speed stock is supposedly 88mph.  You can definitely make it faster if you want.