It's not a difficult job to drop the sump and change the gasket.
Just remember where the short screws go!
Make sure that both faces are clean and free of oil, and being an old git, I would go for the cork gasket. Smear it with clean oil before fitting it, and you shouldn't have any problems.
When tightening it up start in the middle and work your way opposite and round, if you know what I mean, and then nip them all up again in the same sequence.
I would not recommend using the old oil.