Not only are the blades replaceable if you have a blade-plate design; they also swing back when they hit something, which makes engine damage (such as flywheel key-shearing) less likely, and may reduce the severity of some types of injury to people, though the injuries can still be very nasty.
Generally, B&S engines have just a very few standard crankshaft-extension sizes. The main variable is the number, size, and pitch circle diameter of the blade plate attaching screw holes in the attaching boss, which is pressed onto the crankshaft extension. This means the boss can be removed, and perhaps replaced by one that suits a particular blade plate. Anyway, the first step is to report the standard dimensions (crank diameter, number of screw holes, size of screws, and pitch circle diameter for the screw holes).
