Joe that sounds like a real gem of an old tin can you've got
I'd pull that old drain and just install a new one...use 3M 5200 marine sealant to make sure the new one won't leak. The new plugs aren't too expensive.
What is the hull rated for as far as HP?
Post some pics if ya get a chance we'd like to see the boat
Oh...also...welcome to iboats and the tin can club!