Hello Will,
Sorry to hear of your problem, but you are at least lucky that the oil has remained in the sump.
Would I try using an oversize bolt to cut a 'new' thread in this critical application? No, I wouldn't. Would I use a genuine thread tap to cut a new thread? Again no, since a good quality new tap would be fairly expensive to buy, you wouldn't have any guarantee of tapping it at right angles to the plug face (with the possibility of having weeps), and you would likely to be able to guarantee you are going to get swarf in the sump!
Sorry Will, but your action would be a classic from the Bodge-it and Leg-it handbook, and is likely to stand a risk of depositing 5litres of oil on the highway.
Solution:- Buy a new aftermarket sump. Available from Europarts and others? Now that's a proper job.
Kind regards,
Gareth.