Just to say be aware that track sprockets won't fit onto some bmx hubs, or to be more specific, they will fit but bmx hubs aren't reverse threaded so you can't add a lock ring that keeps the sprocket on. You can, of course, make a 'suicide hub' this way using loctite or similar metal glues but this doesn't come recommended.

I built my own 406 fixed wheel after tying out a suicide bmx myself with a proper track hub - something your local bike shop might find amusing but should be able to do nonetheless.