You will need the model number to get the type of motor to fit the mower deck. You will have to remove the carburetor cable and power stop cable.Now on the bottom side remove the blade and blade holder from the shaft. Remove four bolts from deck the motor is off. Just reverse to put new motor on.
Remove the blade and the blade mounting hardware then remove the bolts mounting the engine to the mower deck. Remove the engine then replace it with another.
You can purchase a replacement engine from the manufacterer or a small engine parts store. Usually the cost of replacement is almost as expensive as just buying another mower. Buy a complete engine and not a short block.
No one in their right mind would replace a push mower engine. It simply is not worth the time and cost to go through such an effort. In the long run, you will be happier buying a new lawn mower to replace the broken one.
Turn the mower over and remove the blade. Disconnect the controls. The engine is held on with a few bolts. Remove these bolts. Bolt the new engine on. Connect the controls. Reattach the blade.
Once you have the new engine, it is quite simple. First flip the mover, and remove the blades. Remove the bolts around the base of the engine. Remove engine. Fit the new one in, and reassemble.
You can purchase a replacement engine from the manufacterer or a small engine parts store. Usually the cost of replacement is almost as expensive as just buying another mower. Buy a complete engine and not a short block.