a simple way to fix that would be to take a piece of flat stock the width of the handle base.
cut to your desired length, and round the ends over to your desired shape.
drill the center hole to the size needed to "lightly" tap/press over the hex of the screw thread.
drill and tap the appropriate size threads needed to lock the handle to the hex.
use a common allen bolt [1/2 x 13 x ??? long maybe ?] of a comfortable length to use as a handle.
drill and tap the [handle] end of the bar stock for the allen bolt threads.
cut a piece of pipe to fit loosely over your "handle" and thread the allen bolt into the end of the flat stock until the threads bottom out on the flat stock and the pipe spins freely. some blue locktite is a good idea. cut off the threads sticking past the flat stock.
position the flat stock over the hex, tap into position and retain with the original screw shaft nut and the additional set screws.
done.

i've made handles like this many times for many applications, due to needing them and not having the time to wait for a replacement handle, or a replacement not being available at all.
this sounds way more complicated than it is, but is a very simple thing to make, even if one just has a few tools at his disposal. [hacksaw, drill press, tap and die set, a grinder of some sort]
hope this helps.