I have never tried to remove the point braker plate, so I have no trick for doing that. I had a look in my "Instructions for the operating and care ...", issued July 1, 1923, and my "Repair Manual 1918-1924" (490 and a couple of other Chevrolet models), but I only found instructions for retiming the distributor. Maybe you can find a manual for Delco distributors from these years.
If you have not got the handy tool, described in the "Instructions..." and the "Repair Manual" I can recommend you to buy one. Gary Wallace, Early Chevrolet Parts, offer them.
My tip is that you also make your post in the 1912-1928 forum; that forum is far more active than this 1916-1922 forum. Lots of very experienced and clever guys!
You are right being careful handling parts made of the zink alloy, or pot metal, that is self-destroying by age. Look out for small cracks! Gary Wallace offers new production generator rear end plates and oil pumps, but I have not heard of new distributor housings.