All right guys, since no one else chimed in, I am going to go ahead and answer my question. The lug nut size for the UXV 450i is M10x1.25.
That was what I came up with in my online search, so figured I'd just order some and see if they fit. Sure enough, they do indeed. Replaced that lug I lost and now have three spares for if/when I need them.
Hope this information helps someone else down the road. Happy off-roading!