G'Day lwakelin .
You want to buy a 1/2 Inch BSP pipe plug ,you should be able to get one from a plumbing supply shop maybe b and q make sure that the plug is SOLID that is some you can buy are semi hollow you want a solid one Brass is best,borrow a 1/2 inch tap from maybe a plumber or your local engine rebuilder.
1/2 inch bsp has a tapered thread and is known as a dry thread seal that is the fact that the plug in this case, is tapered, 1/2 inch bsp measures 1 inch diameter.
Clean the thread out with the tap screw it in removing every time checking and you will see the new thread appear tap only deep enough to clean the thread no more ,then try installing the plug DRY it should go in AT LEAST THREE TURNS,when you have tapped it out enough smear the threads with loctite thread sealer and install the plug ,it does not have to be WHITE NUCKLE TIGHT Just enough to seal, fill the engine with coolant and run the engine up to operating temperature ,check for leaks and if all o.k cut the plug of flush with the top of the cylinder head.
job Done.
Happy Days.
Hutch.