coolant drain or oil drain? oil drain is low pressure (I would say no pressure, but on a diesel with blow-by possibilities, low pressure) round ball looks like an oil galley plug, and yup, looks like a heat shield
yeah, that is the "hydraulic head" oil drain I wouldnt THINK it would be a problem?
someone else would need to post up! me personally, I would crack open the aviation form a gasket and put a dab ONLY where the different surface is and RIGHT before assembly. I wouldnt put it on the gasket, or on the other side, just a tiny bit right there on the ding, but someone else on here might be able to give you more opinion. also there is still quite a bit of material thats NOT damaged, so might just be able to run it the way it is.
I think you could get away with it, as mentioned, it's not on the pressurized side of things.
If it bothers you then have it welded up.