I've fixed at least 15, maybe 20 of these in the past several years. After you removed the old plug the lead thread was damaged. Buy, borrow or find a buddy who has a 1/2 NPT tap; carefully start the tap in the threads and gently chase the threads. Be careful not to start cutting new threads and you'll be able to screw a new plug into the fitting.. Be sure to use Teflon tape.
I actually keep a couple of 1/2 NPT Nylon plugs and a tap in my tool box, I had to fix a neighbors for him on my last trip.