Can you please elaborate? Not sure if I fully understand your question.
If I am reading it correctly - for doing this in CAD software, I am not sure if it makes any difference:
Make the body → put the holes → make and combine the sprocket hubs and the rest vs. make the sprocket hubs → make the body → make the holes and then combine the two parts.
I agree that Tamiya, Academy, and ESCI/Italeri sprockets are all inaccurately shaped to start with.
The most tricky part is making the sprocket hubs with the correct spacing, which is going to require some math: algebra and geometry. 
I’m putting that off until later.
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