There are various size plugs made just for this eventuality.
Many parts stores have them and the taps necessary for the job.
The taps are a special type that does not create cuttings. It sort of stretches the metal to create a new thread rather than cutting the metal to make the new thread. The noise the tap makes when it is being turned sounds like strangling a cat.
I have borrowed them from shops in the past (e.g. Rare Spares stores).
as well as from service stations (the old type where they have real mechanics doing repairs).
The servo suggestion is the best one. Many servos where there are mechanics or car repair places will have these and would cut the new thread and supply the plug. Takes about 15 minutes to do and it should be only a few dollars.
The XR family ---->
Muzza: a XR8 AU seriesIII...bog standard :)
TBH: (the better half) drives an ED XR6 wagon (Mock Dev 4 160rwkw, JMM extractors, 4 electric windows, Ghia door panels in XR trim, rear head & arm rests in XR trim, Momo s/wheel with Cruise, EL thermos, EBGT wheels, etc) things to come soon = EECV upgrade.