There are two common causes of oil passing through the breather. The first is over-filling the sump with oil. The second is combustion gas getting into the crankcase under pressure. There are three ways gas can get past the piston: a hole in the piston, piston rings not sealing to the cylinder bore, or in the case of OHV engines, a blown head gasket.
I suggest you begin by checking the oil level, and verifying that you haven't been trying to run the machine on a slope of greater than 15 degrees. If that doesn't provide the answer, a compression test may be useful, but probably won't in this case if the previous tenant actually did replace the piston rings. If he didn't replace them, you'll probably find that the compression pressure rises substantially when you put a spoonful of engine oil in through the spark plug hole and repeat the test. This would mean that the piston rings are not sealing to the cylinder bore.
If it should turn out that the oil level is correct, you haven't been operating on a steep slope, and the compression pressure does not change much when you put oil in the cylinder, the remaining strong possibility only applies to OHV engines like yours: the head gasket may be blown to the pushrod cavity, which pressurises the crankcase and has the same effect as blowby passing the piston rings.
Please let us know what turns out to be the problem, or ask further questions if necessary.
