This is another application more suited to a stud and nut rather than a bolt into a blind hole. A stud saves the wear on the alloy thread if the unit has to be removed several times. There is usually enough space to drop the box/engine unit down vertically on reassembly, where time is on your side, rather than the throw it into the frame modus of the production line.
The original thread would have indeed been BSW..."Whitworth" a thread form near as dammit interchangeable with UNC, with the exception of 1/2", the TPI is the same. Modern replacement primary and timing cover bolts are UNC in my experience. Thread angles differ, but this is not of consequence in a worn alloy female thread.
Alloy needs a coarse thread to give the support to the fastening, BSW and UNC are chosen for this reason and I (hooligan) would have no problem using a standard Coarse Metric thread in this case. Certainly easier to source a helicoil to suit. Sometimes simply using a new bolt does the trick, as does making sure the hole is cleared out and the whole of the internal thread is being used, so correct length bolt that does not bottom, or a stud that does.
Tin hat on, under the parapet ....final choice is yours.
Swarfy.
