I am wondering that if the housing's inside surface was scratched and damaged by the rotors, can the housing be rebuilted or reconditioned? or need to buy a new one?

 

any deep scratches on the rotor housing and they will need to be replaced. if not they will loose compresion through the grooves and wear the motor out prematurely. if they are not very deep then you could use them again but you may have an issue with low compression. I have this same dillema right now, if only the housings were not so damned expensive.