Small Engines (Lawn Mowers, etc.): John deere 345, blown head gasket, john deere 345

Question
Hello. I have a JD 345 20 hp Liquid cooled engine. currently 500 hours/ 10 years old. It is used year round, mowing in summer and plowing in winter. I change all items in the "maintenance kit" as suggested by JD and do lube, oil, and filter as suggested. Recently the tractor has had a hard time starting, I must hold the choke on for 10-15 seconds and when started, white smoke emits for about 25 seconds. Additionally, when running the mower sounds like there might be a "drag". The pulleys and pivots points were just lubricated(not sure if these 2 symptoms are connected). Any ideas or suggestions would be great!

Answer
White smoke = burning oil.  80% chance it's a blown head gasket.  The other possibility is a dead coil.  Pull thre plug wires off one at a time while the engine is running.  The engine should change tone but stay running.  If you pull a wire and nothing happens that is the bad cylinder.  If you pull a wire and the engine dies that's the good cylinder.  Test for spark on the bad side.  If you have spark, pop the valve cover off and make sure the push rods are in place.  If you have spark, and the push rods are in place you have a blown head gasket.