All model numbers for Briggs and Stratton engines is stamped into the valve cover on the engine. Also on new engines their should be a sticker on the back of the engine.
The model number will be stamped on the lawnmower near the spark plug. Often is it on a metal plate near the muffler. It can also be stamped on the metal shroud. The number will have three sections. The model, the type, and the code. At briggsandstratton. com you can find documents and videos that will help you locate it.