Q: Plug no longer works can it be replaced

See Find the right power adapter and cord for your Mac notebook - Apple Support  for the correct power adapter, just the plug cannot be replaced.

Ok, if the duckbill is all you need, try an Apple Store and see if they will sell one.  Normally those parts are not stocked for sale but give them a try.

As I said, I have never seen them in an Apple Store, but it is worth the effort to ask and see if they will sell one.

The Apple on-line store does not show them as available.  You could try Google to see if anyone is selling them, or places like iFixit.com

In bins at second-hand stores, I've found the duck-head parts as well as the official extension power cable.

The cable works better to keep the AC adapter plugged in, but isn't as portable to carry along when traveling.

If you happen to have more than one power adapter, a long cord could remain with the unit used at a desk.

The early AirPort Express wireless unit also uses the duckhead or power cable interchangeably so these are

also sources of those parts. I have spares because I always look for these Apple accessories, when I don't

need them. Later on, various adapters and other bits are helpful; some are obsoleted and harder to get new.

https://duckduckgo.com/?q=apple+duckhead+adapter&t=ffsb&ia=products

There are sources online, depending on where you are located globally; regional sources may be able to ship

further or overseas, but not all can at an affordable price. However I've never ordered these, & prefer to find

them randomly since I frequent second-hand stores to look for collectibles or useful items, tools, etc.

Good luck & happy computing!