Blowby, is caused by worn piston rings, where combustion pressure is blowing by the piston rings, and pressurizing the crank case, theryby blowing oil out the dipstick tube.
If the tractor sat for a year, there's a chance that the rings are rusted, and not wearing evenly, or a cylinder wall is rusted and not letting the rings seal, thus letting the combustion air by.
First step, go buy some Sea Foam, add it to the crank case oil, and to the gasoline (according to directions on the bottle), and run her hard for a couple of days.
Go mow with it, and put her to work. MAKE SURE THERE's THE CORRECT AMOUNT OF OIL IN THE CRANK CASE, and check it often. Sometimes this will help break up carbon and crud in the combustion chamber. If it does not clear up, you can keep running it, but eventually you will need new piston rings and an engine rebuild.