The prop nut and prop washer that come with the engine are EXTRA hard so they can take the tightening they need to NOT come loose like yours did. USUALLY the parts are caught inside the spinner when they come loose but you need to be careful not to drop in the grass or let them fly away. Check with your LHS and get the correct parts. Part numbers are in your engine manual.

Fotr the weekend - I believe it's a 1/4 x 28 nut.