Whats normally in it? A Banjo? Whats in it now? A straight fine-threaded plug with a little copper gasket would be easy and clean. If you try to jam a MPT plug in there the threads will be ruined immediately and possible cracks and damage to the passage in the head as well
Sounds classy. A real Harley guy. Haha.
The right way would be to take the tube out, tap the hole and seal it with a plug and a gasket.
The easy way is to stick something other than a random bolt in the hose. Bolt threads aren't meant to seal pressure (obviously now, right?). Use something meant to seal against liquid pressure and will take the heat. A brass barb fitting to cap, a flared tube thats sealed, not a straight rod though, because you need a flare to stop it from spitting out of the hose and let all of your engine oil drain all over the exhaust and rear tire.