One of the crankcase screws/bolts must have come out, the oil level never dropped when I was running it but I would like to get a new one, what size are these? I bought this used about four years ago, here's a link to a PDF file, page 8.........part number 22. http://bsintek.basco.com/BriggsDocumentDisplay/Z6hssAVJ1DajI.pdf
Pull another one out and see if it fits in the spot you have missing. Then match it. It's nothing special, a regular hardware store bolt will work just fine.
possible metric when you go to hardware store. Real helpful PDF doesn't call out the bolt size or thread, lazy sob's, or just greedy charge ya $3 for $.25 bolt.