i ordered a NA capri radiator online, spent a great deal of time ensuring i ordered the right one, i saved a mint compared to other online stores..and they shipped something that was not as-advertised..
i shipped it back, went to a local radiator shop, and got the right radiator for not much more money.. i only had to wait a day or two.
as far as your plug.. it's plastic, i once did the same thing on another model of car and believe i unscrewed it by carefully drilling down the center with a drill bit (in reverse i think).
then i added a slightly larger drillbit and drilled into it in reverse to unscrew the old plastic threads.
i've also done this - if i can do it, anyone can:
if you can get a replacement plug, determine what size of bit you need to drill into your bung without hitting the threads, you may be able to patiently pull-out the plastic threads from the bung threads with a paper clip hook.
i do all this work on my back with the radiator in-place on the car so no foreign objects fall into the radiator bung.