Looking at the pics, in my opinion only, it is not the seal or the sump gasket. Looks to me like the engine had been sitting or used in a dirty environment and had an accumulation of dirt and oil all over it, which would not be uncommon. Before I would pull the unit apart, I would clean everything up, reassemble and mount it on your unit. My system for finding oil leaks is after cleaning and drying I spray flour over all areas where I feel oil may be coming for. Start the unit and run it for 1/2 an hour, then let it sit. If there are any leaks you will be able to trace them to the source. May be a bit messy, but will solve your problem.