what kind of axle does the positron have?
Do you have one? What does it look like.
It’s most likely a set screw. Likely sizes are either a #6, #8 set screw or a 4 mm set screw. It’s most likely a 4 mm. Take it to a hardware store and see what fits of the right length. If you can’t find a set screw, get a normal screw of the proper size and cut the head off. Smooth the cut end with a fine file or a coarse sharpening stone.
The Positrons axle is similar to the genesis axle, a YYF 12mm will do.