I try to model the complete part as it is moulded. But when I have a damage an expensive part, then I try to ask some of my friends, if someone has the part with scratches or evenly completely damaged. (See how the the 9V Train Speed Regulator was done and note that the inside is still simplified.) And if it is not possible to get an damaged part, then I would go the way an model only the outside and make an 'fake' inside, so that at least the part is closed and has no gaps.)
/Max
/Max