Ive had a dropped valve punch through the cases on my cb550k3 engine, luckily it was by the starter motor, so I just smeared some solicon sealant around the hole, bunged the metal bits back in place and clamped the starter over the mess, sorted the dropped valve with a crumby used head and kept riding, the k3's original engine had suffered a snapped camchain, something of a jinxed bike, that was, but the bodged engine kept going for years before snapping camshaft.
