You can plug the hole with a bung of some sort, even a cork, then observe for any leaks. However, it should be relatively easy to get a replacement - a second hand one can be had from a vehicle dismantler or scrap yard.
Would you be talking about a Pug by any chance? the base of these stupid things have a tendency to snap and drop into the sump, Put a plastic bag over the oil filler hole & secure with a rubber band.