8-32 socket head screw. You can get a longer replacement at your local hardware shop, but you might need to file it down so as not to interfere with the battery pack.
As for the step in the middle of your wheel, you can smooth the hilt by sliding a shroud over the area. A ring cut from 1.5" 22 Gauge sink tube will just side over the hilt. Alternatively, thinwall 1.25" PVC will also fit, but not as snugly.
if the screw hole in the hilt has been stripped, you might need to drill and tap a new 8-32 hole elsewhere in the hilt.
Finally, it sounds like you just recently ordered - this sort of thing is probably covered under your warranty, if you want to shoot an email to
[email protected].