You need to be finding out WHY it has no frame number to start with. If it was bought new unregistered it would still have frame and engine numbers, just not given a reg number.

It its not got one, and its definatly not been 'ground off', then it must have had a new frame at some point so obviously its been bouncing down the track at some stage. Nothing to worry about aslong as its straight now though. id be very wary though if the owner says he had it new with no numbers on though.

As said though, pleanty of nicked track bikes about, and im sure plenty of nicked track cars with no ID about too, but id personally want one thats got ID so you know for a fact your not riding around on a dodgy bike thats effectively someone elses.