Either UNC or Whitworth would work equally well. I take it the lock is on the left side just below the round plate. Making a handle out of a suitable machinable stainless steel would be best, it can be threaded both ends and then bent to the right angle afterward, and the clamp nuts can be made from brass as one piece before producing the curved ends to match the diameter of the quill and then cut. Only the right hand nut has the thread, the left one has a clearance hole
Lathes UK website have lots of info and pictures of Elliot mills and probably sell a manual which would be useful.