Thanks to all who responded; the specifics cited by Christopher, Fotch, and George are especially helpful.
I checked the screw earlier this morning; now that I'm not relying on my questionable memory, I note that what I need are apparently 6/32 x 1/4 machine screws. I'll try Lowe's and/or Home Depot later today. If I have no luck, there's always the mail-order alternative with some small-parts supplier.
In over three decades of using my 45 MCR-X, i've lost only the one screw, so getting two or three should be a permanent solution.