It might be the crank seal (PTO side or flywheel side) or the sump cover gasket. How bad is the leak? I had one that leaked a little at a time and using Rotella 15w40 pretty much stopped it. I also had a 5hp Briggs with a ton of hours on it that basically poured oil out of the seal.
The crank seal and sump cover gasket aren't difficult to replace, but it requires unbolting the engine from whatever it is mounted to.