Hi Adam,
The original manufacturer owns the design rights to the shape of the car. This is automatically granted as soon as the design is produced, but can be registered as well. You will find more info at
www.patent.gov.uk, look for information on registered designs.
As far as I can tell, manufacturers aren't so bothered about models that they don't produce anymore, but kick up a right stink if you copy something they are still selling. For example, there are plenty of people doing Countach and Diablo replicas, but anyone trying to do a convincing Murcielago rep has been shut down by Lambo's lawyers. Although that may be down to their new owners, VAG, who may have decided to take a more aggressive stance. That's why any Murcielago reps around are different enough in detail to not fall foul. Unfortunately it means you can tell they're replicas. As far as I know Ferrari take a very dim view of copying of products past and present.
If you don't want to say what you're making a replica of in a public forum, feel free to PM me if you have nay other questions.
Regards,
Justin