That cotter pin is captive: it cannot be removed, it can only be loosened then the clutch is slid off the crankshaft extension. (It is often necessary to soak the crankshaft extension in Penetrene for a few hours before trying to move the clutch off it.) The replacement clutch-half will not have a captive cotter, it will have two setscrews at 90 degrees to each other, which is much better. Also, when you buy the new one you will have a choice of 5/8" or 3/4" shaft diameter. Just be careful to buy an engine and a clutch-half that match each other.
The Briggs 550, being OHV, will use considerably less fuel than the side valve engine it replaces. It should also run rather more smoothly, since it will not miss shots all the time, and idle roughly.
