This mower was made with at least two different engines: the 350447 Vanguard and the 422437 Opposed Twin. These engines will not replace each other. If you have an Opposed Twin, you will need to use our Briggs Vanguard conversion kit. If you have a Vanguard engine, you need to go back with a Vanguard engine, not a conversion kit. You may have to change minor parts from your original engine to make this work the same as your current engine. You will need to use the drive adapter and muffler off of your original engine. You may have to use the oil filter bracket as well and the wiring could be slightly different. If you drive off the flywheel side of the engine and your engine uses a Booster Fan like our part BS-690379, you can use any of the engines listed but you will have to use your flywheel, shroud, and starter or starter gear from your old engine. You will need to match up your starter to the correct flywheel, eg: aluminum gear on your flywheel to nylon gear on starter or steel gear on starter to steel gear on flywheel. The 356447 series engine is about 1-3/8" wider than the 303447 or 350447 series. 16hp Vanguards have an all cast flywheel with the fan as a part of the flywheel. The 18hp and up Vanguards have a cast flywheel with a plastic fan that bolts to the flywheel.