Going back many years, but I built a Triumph Trident with a 'factory' frame which had no number. Getting registered as a new road bike was no great problem but the frame number - which I devised and included my initials - had to be stamped into the metal of the frame. A riveted on plate was not acceptable.
Probably bears no relevance to today's regulations in South Africa. 
Any frame/chassis number has to be stamped into the metal of the main component, iow not the subframe. A metalplate with the number is allowed, but only in addition to the frame-stamped number.
I better get busy with the stamp set then, plus a plate perhaps?
What frame did you use, a Rob North race replica or something? My brother used to race a Trident Triton, sounded fabulous, my friend has a Rob North Rocket 3, it's pretty cool...