There's a weary old jargon term in CAD, "design intent". When the toy's assembled, you know where each part fits, its relationship to the parts that are attached to it and their place in the final assembly. There are some ruling dimensions, the wheelbase, the diameter of the axles and bearings, the thread for the neck of the bottle, but for nearly all of the others, the dimensions are relative to each other rather than absolute.
I've been involved with reverse engineering for a long time, ultimately you need to determine a resolution that you can live and work with, whether it's determined by the resolution of your measuring tools or by the means by which you'll produce the clone. For additive rapid prototyping and 3D printing, layer and filament thickness mean that these will usually be coarser than subtractive means, CNC milling for example. For the toy, the only way you'd get a resolution that makes a copy indistinguishable from the original is to use the parts as a master for a steel epoxy or electrolytically deposited nickel cobalt mold.
The short answer: I'd settle on centre-to-centre dimensions for the parts with circular features, face-to-face relationships for the parts that align to each other, get the angles between the faces to agree (bearing in mind that they were originally probably designed with sensible angular figures, whole degrees or down to .1 resolution rather than arbitrary increment angles) and be very satisfied with making them look as similar to the original as possible.
Scale modellers who show their finished miniatures in shows gripe about judges who reckon they know about every nuance of the full-size prototype, those judges are called "rivet counters" because there have been occasions when they've asserted that the number of rivets in an aircraft panel's edge is wrongly represented (maybe they have, or maybe that's apocryphal exaggeration, but you get the drift), and there are people who build the miniatures who claim that they've exaggerated curves because it "corrects viewpoint perspective distortion". People like that can drive you mad, you don't need to do it for them...