what?
fixed gear: bike with fixed rear wheel
singlespeed: bike with rim or disc brakes that has a freewheel or freehub
coasty: dumb way of saying bike with coaster brake, which is a hub with an arm activated drum brake.
unless you mean singlspeed, which if that is the case all you need is a set of brakes and a bmx freewheel to thread on where the fixed cog should go.
however, it depends entirely on what your wheel has on it now and what you are trying to do. if you want a coaster brake where you stand on the pedal as it's behind the crank to brake, then you'll need a new hub.
hep me!
what the **** are you on about?