Make sure whatever you use to plug this pipe doesn't blow-out due to positive crankcase pressure. I can tell you from experience it's a real mess when you puck
oil out of an unplugged tube under full power! Obviously a fire risk too! After my mess was resolved, I used a 2" piece of 3/4"
oil hose, a short piece of 3/4" solid Alum rod as the plug (or other copper fitting, per ideas above, etc) and two worn drive hose clamps. No problems since. I eventually plan to add a puke tank to replace this plug. There may be a more elegant solution out there, like a brass compression nut, etc that would fit over the tube O.D and seize down on the OD of the tube. A few minutes at a local hardware sore, or maybe Earls may yield some new ideas.