Never made a water pump, mostly just bored and thought I'd type something ...,
At this point it probably would be better to be looking at the pump part engine block (because that is the part you are trying to reproduce).
I would make as many measurements as possible, of course paying particular attention diameter & depth of pump cavity - (is it round?), o-ring and stud positions, any "swirls" in the bottom, etc. But don't overlook to inlet and outlet designs and positions as well as length and diameter of immediate straight sections of inlet and outlet paths.
It will probably work pretty good without being an exact copy of the engine block, but the closer the better! I believe (unlike many others) that most times that the engineering the professionals produce is probably a good balance between longevity, function and cost. You might make it work a little better, but you have a lot better chance of screwing it up!