I think that the screw used in the late 10" and 12" half-nut lever is an oval head flat head, AKA a rack and panel screw. I have them in #8-32 and #10-32 but only in a length that you would probably have to cut off. However, the late levers with the countersunk hole are dome shaped on the casting. In your case, a standard flat head would probably look better.
For the record, I no longer comment on the 10 million colors that people have repainted poor Atlas lathes. All of mine will remain the original colors, which at this printing is Chinese OD, Shopsmith Green, Onan Green, and Atlas gray.