with the new SEVS regulations, the parts have to be removed for compliance. these parts can be refitted after the compliance process. To my knowledge, the only item that has to be destroyed is the WHOLE car if it does not meet compliance (i.e major accident damage or corrosion), it's either this or ship the car out of the country and show proof (of either).
You are able to bring in a car that is modified, but generally the cost of obtaining the original parts and then paying someone to remove and fit is the killer on the hip pocket. I have looked VERY closley at the DOTARS legislation, as I am awaiting the outcome of an Ombudsmans decision on the 15 year old rule (I bought a car mid year and now I'm ona knife's edge to see if I can bring it in) any q's, let me know.
Cheers