I vote for replacing it.
So long as you are asking, I vote for replacing the through hull and keeping your sink functionality. I sink provides a lot of convenience and utility, even if you do not use it that much. If you want, you can only use the sink for things that produce relatively clean water afterwards. Besides - think about how much harder the boat will be to sell with a non-functional sink. Will prospective buyers choose your boat when they have other choices where everything in the galley works?
If you decide not to replace the through hull, then I'm with everyone else - you need to grind at 12:1 and glass from both sides. When one buys a used boat, one depends on previous owners to do the right thing with maintenance and modifications, and surveyors don't always catch everything when they don't. A glued-in plug will be much weaker than the rest of your hull and less resistant to shock forces than a proper repair would. Who knows what could happen with that? In 10 or 15 years, the boat owner could be dismayed to discover that his boat is shipping water at a prodigious rate when he bumps an underwater obstacle and the plug breaks loose because the ageing epoxy or glue failed.
T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama