The crankcase itself is cast aluminium - it is not brittle, and would not shatter on impact. Usually the bottom of the rod separates from the crankshaft at the top of the stroke, and at that point the rod has a very high lateral velocity. The bottom of the rod is hurled sideways, and hits something before it stops moving. The something might be the crankcase, or the camshaft - and either way it will cause damage. When a rod breaks, if you were paying for labour, you wouldn't pay - you'd scrap the engine. It has to be examined carefully for signs of impact damage, and the damaged parts replaced. Chances are the inside of the crankcase has been hit hard, and this may have distorted it, which would make it more or less useless. In addition iron parts that have been hit may be cracked, bent, or both.
As a practical matter, you may be able to make a hobby job of this. The crankshaft may be OK. If you are rather lucky the crankcase may be OK. Most of the other bits may or may not be the same as corresponding bits of the 9 cubic inch engine used in lots and lots of vertical spindle lawnmowers - I don't know, but Joe or Bruce will. You can probably pick up a 9 cubic inch engine on a nearby nature strip.
