Thanks guys!
I have not checked the internal threads of the suspect port, but have on the screw. They, as those for the mounting studs and nuts and the brake line fitting bolt and nut are SAE.
I will look for a plug, as suggested.
If the remedies suggested fail I wonder if using a short set screw and then plugging the top several threads of the port with solder would work? Before doing that I'd disassemble and clean the MC. I want to avoid piston damage and a small exciting fire otherwise.
Another option could be to drill and tap for a replacement bolt.
The tape idea could work, but from what I've read if a bit of that gets into the system it could plug a line and cause the brakes to fail.
