so, if your tap screwed in fully with no unusual interference, simply get a 3/4-16 bolt with a flanged head and use a plastic sealing washer under the flange if you are unable to find a proper plug anywhere. barring that gasket idea, a simple home made oil resistant rubber washer would work or even a copper ring. try a big truck parts store, like a truck stop if you have them, and see if they have a washer to fit. they may even have a plug with that thread, who knows. some will even have a magnet.
https://www.dormanproducts.com/p-21570-090-008.aspx
https://www.summitracing.com/tx/sear...size/3-4-16-in
https://www.napaonline.com/en/p/NDP7041015
https://shop.advanceautoparts.com/p/...226/17190131-P