Measure the I.D or the threaded hole, which will give you the minor diameter of the hole.
Now--take some modeling clay and press it into the threads , and on the clay, measure the pitch with a metric dial indicator.
Or, easier yet, take a thread gauge and measure the pitch.
Now, go to a copy of Machinery's handbook and look up the depth of the cut, i.e, root depth.
Now the easy part--minor diameter + 2 times root depth = major diameter.
Lathe turn and thread accordingly.