Troybilt lawn tractor - "Super Bronco" mower dies. I pull plug off crank case and a quart of gasoline/oil mix comes out. How is the gasoline getting into the crankcase?

Oil breather is broke.

Usually replaceable from the outside of the block. Usually there is a hose to nowhere attached to it.

Other possible reason is that the fuel valve is sticking because of being stored without any kind of fuel stabilizer for 3 months or more.

Both can cause it. The former is a slow process. The latter is not.