Can you specify which model bike and which display, if one of the newer 2-screen models? I switched to the aftermarket 35A display/controller last year, so can't specifically answer your question, unfortunately.
Whenever I have an issue like yours, I just take one of the remaining screws to my local hardware store and match it to something on their shelves. Always test-screw on a nut as well to confirm you have the right thread pitch in addition to size. Depending on the stock screws, this might be a good opportunity to change them all out to a high-quality stainless-steel version if appropriate.
And don't forget to buy one or two extras for the next time!