Hah, I did the same thing. I printed a replacement plug, it was super easy to design. If I can find the file, I'm happy to share it if you have a way to print it.
If/when you plug the drain, you'll need to address the return flow. I had to crank mine down to a ridiculously low level in order to match the water volume that's able to get through the overflow. To compensate and keep my SPS happy, I had to add another powerhead... all that said, I've been running like this for a couple months now and all my problems related to water level, flow, filtration and salinity have been fully resolved.