wa is more lenient than other states , but that doesnt mean that the car was modified like that when it was registered in nsw though , it probably wasnt.
exhaust is ok if not to loud , dont know about the no horn though , youll probably have to do something about that .
to reg it you rock up to the pits with your current proof of ownership and registration and they check it out and give you your wa plates and reg if its cool and you pay the fee