If you mean the big 'ole on the bottom right of the sump
not the spacer/flange I think
but I'm working from memory here, that it is an orifice through which some other machining operation is performed which is then blanked off afterwards.
Anyway in all real terms it is functionless and it is probably blanked off with some sort of threaded plug. Best way to tackle a leak would be to clean the area thoroughly, preferably with the sump off on the bench, in some sort of aggressive degreaser and then spray clean with first brake cleaner and finally contact cleaner. Then apply some Loctite 'Super wick-in' and let it weep into the threads before finally wiping off the excess and then either fill the hole with silastic or a rubber bung sealed with silastic.
Well that's what I'd do.