Yes, but I doubt the rebels have a lot of parts on hand that would fit the Falcon. (We never see them using a similar ship.) Plus, it's hard to swing by the local intergalactic Pep Boys when bounty hunters are out looking for you.
In one of the Brian Daley books, Han and Chewie had to make do with parts that really were inferior, as there was nothing else they could get. Han even expresses his desire to rip all that stuff out at the earliest opportunity.