Paint Removal: The Issue
The original paint work on standard die cast cars can be quite thick in order to withstand the rigours of play. Although you can spray directly over this paint work, stripping the vehicle back to the bare Zamac will preserve more detail if you want to repaint it.
A general purpose decorators paint stripper will generally remove the paint from die cast cars without issue. Thicker gel like solutions are less likely to evaporate and are easier to manage in case of spillage. The active ingredients and strengths vary depending on the product, always read the label and follow its guidance. When the stripper is decanted into an unlabelled container consider labelling it clearly with its contents and appropriate warnings for easy identification by others.
Note: this process is only suitable for metal components, plastic components will quickly soften and degrade in most paint strippers.