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If the original 1/8″ BSP thread for the drain tap has corroded badly, and is not recoverable, you will need to go oversize, and as 1/8″ drain taps are easy to find, the best way to go is to install an adaptor sleeve into the oversize hole, with a 1/8″ BSP hole to take a replacement tap. This keeps things ‘standard’. Be VERY careful when drilling and tapping the oversize hole, as water jackets are surprisingly thin, and always corroded internally. Some castings have the drain hole lower than others, and these are usually OK, but some have it, for no good reason, about 3/4″ higher up, where the metal is thinner and more fragile. Why this was done is arguable, but it means that after draining down, a fair amount of coolant is left festering in the water jacket, causing corrosion when left standing.