Bonny - grab yourself an M4.5 X .75 die. This will be close enough for this job. It's nominally about .180", but after you've threaded the shank in the lathe, flatten the thread crests off a little with a file, and run it in and out of the lock plate with a little oil. The plate should be hard, so the threads will iron down well.
This "swage fitting" will match the diameter/thread angles/ slightly different pitch up well enough. Don't worry about the thread angles on a job like this - you're not building a nuclear power plant.
The bloody Poms were all over the place with their threads. I have replaced a lot of screws with the above die - most matching perfectly.
