thanks for your helpful response. The CAD model is looking decent but I don't know how to measure the bolt-hole pattern for the base of the distributor cap that bolts down to the rest of the motor. Are there any drafting tools I can buy to find the exact center of the distributor cap's holes and their displaced angles?
Try measuring the ID of the plug holes and then take a pair of calipers and measure the OD of some bolts. If you are lucky you will find some with a snug fit. Chop off a longish section of the bolt and chuck it up in a drill press. Run the press and take an angle grinder and create a conical point. Remember that three points determine a plane so take three of these tools you have made and install them at the same relative height in the cap every 120 degrees.
Print out a diagram of the six plug holes with their centers marked and an array of evenly spaced lines bisecting the center of the major circle on whose perimeter the centers of the six holes lie. Add another major circle that tangentially contacts the outer rims of the plug circles and a third major circle that if you cut it out with scissors will allow the outer pattern to fit over the distributor cap without interference. Unless you have printed to very heavy stock paper you may want to transfer/glue the design to flat cardboard stock. Do not trim away the outer edges yet. Prick through the centers of the plug circles with a pin.
Before doing the above you should print some circles of known diameter on the printer and check the x and y dimensions because often there are correction factors needed.
Ensure that the distributor cap will sit down evenly on a planar surface (a very flat table or a granite countertop). You may have to sacrifice a cap by carefully grinding the bottom so that it will seat flat/evenly on this surface.
Obtain a machinists square. Carefully seat your printout so that the pricked pinhole centers align with the conical tips of the ground bolt tools. Slide the upright machinists square so that you approach the outermost tangent point of the mounting holes in the cap. Now you may find that you will need to trim the printout to allow the square to approach the cap but still allow the upright portion of the square to become a "pointer" of sorts at the array of lines bisecting the major circle. This is like using a degree wheel to check your timing.
From these measurements you should be able to determine the approximate offset of the mounting bolts to the plug holes. You will have gained experience in making your own tools and learned of myriad sources of possible error which you can and should write up.
With a laser scan of the cap you could also do this but save that for later projects. If you do it manually you will remember, probably for the rest of your life, all the issues and possible inaccuracies.