Mine came with a bad black plastic coil box, are these worth rebuilding?
Modern plastic? No.
One of my T's, a 27, has 4 black coils, re-built with new capacitors a couple of years ago, no problems other than a set of points broke in half once.
I do not doubt Frank in "stralia one bit.
Some of the black plastic coils can be successfully rebuilt, but not all. These coils were made in various forms for purposes other than the Model T magneto and may not work in a Model T after rebuild.
Here in America finding good rebuildable Ford coils is easy and is the best way to spend your coil rebuilding dollar.