There is a possibility that the pump housing assembly from Amazon is junk (sorry but you have to look at all the angles) All it takes is some dubious machining and it will haunt you into the afterlife. Now I know that this is not something to recommend but...thread tape maybe? If a strip of teflon tape goes adventuring into the oiling system it could easily cause a catastrophic failure (thus the reason why it is not done in the first place). Is there a torque specification on the plug? Can you try a different plug?