The Model B distributor is probably the answer. Attached is a photo of the one on my Model A engine. The "B" on the head is indicative of the "Police Head" manufactured by Ford for use on the Model A. The only visible difference is the rod to adjust the timing is missing and the condenser location is different. No need to change to 12 volts. You will have to use the Model B timing cover if you want to set the initial timing by the book or there are a couple of "work arounds" in the archives. Get to know Dave Renner (at Renner's Corner, ((734) 428-8424) for Model B engine parts.
As redmodelt said, you should post in the Model A forum section for better results.
Charlie Stephens