Couple of things Chris.. Any aircooled engine that is "hidden" will overheat no matter what. In the photo you uploaded, there is not nearly enough ventilation for the high 3D loaded engine. You need to fully open the front of the cowling and cut a BIG opening underneath it, and use at least 18% of oil, no matter what the quality is..

Secondly, the hole at the rear body of the engine blockyou are talking about is actually an access hole to remove piston pin retainers. It shouldnt leak under normal operation as it is completely sealed with the cylinder wall against it.