My quick guess is that the engine is a Briggs & Stratton model. Look at the engine (rather than the mower body), and somewhere on the sheet metal you'll likely find a few lines of embossed numbers. If the engine is really gunky, you might have to clean it before the numbers show. Once you find them, shine a light from the side and they'll stick out a little better. The top line of numbers is going to be the model number. You can go to briggsandstratton.com to find manuals and jackssmallengines.com to find a parts diagram and part numbers.