I've had very good luck with a cleaning product called Simple Green. In some cases the shell needs to soak for 24 hours. It does not remove everything, but it has removed most paint on Atlas-Rivarossi cars I have tried it on. That said, some enamels of that era just don't come off- so sandblasting might be the only way to go- although the enamel paint might be tougher than the plastic shell, so be careful and use some masking if possible.
The downside is that Simple Green has a pretty strong smell- so rinse thoroughly and prime inside and out, or it will smell very "clean" for a very long time.