Argonauto, Once you have removed the sump (careful not to damage gaskets and seals)
The sump needs cleaning
of all oil
residue - then first examine for obvious holes, then for pin holes that have rusted through from the outside. IF no holes are found check the sump's fore and aft oil seals
The most frequent problem where the bottom of the pan has been in salt water
that has sat in the bilge
is first pin holes, which in time get bigger.
Generally no need to replace with a new sump - take the holed sump to an auto radiator repair shop and they will braze weld it with oxyacetylene and an appropriate alloy rod (bronze + --- flux)
Once repaired, clean up the bilge
the sump with protective paint
and sump. Add oil - check for leaks
Keep salt water
out the bilge.