CEI is less used on the “modern” BSA
’s like the C15.
On C11/C12 there is more because thy were pretty much hangovers from prewar. Your C15 will be unit construction so quite a different beast. A whole re-design of the bike really.
Fork legs I think will still be the BA thread form.
Can you chase the threads with a 2BA tap and a die nut? Perhaps add some thread tape to take up the wear.
After all, it’s just a drain plug and a locator for the restrictor rod.
A screw insert that small might be a fiddly fix ( unless you have the gear to keep it nice and accurate)