Re: A major problem with my new project bike
Try sticking a magnet to the frame. If steel, it can be welded and the 'sleeve' idea is a good suggestion. If aluminum then you'll need to find a competent welder to do the job. Welding aluminum is for people with experience, not the hobbiest in his driveway.
Whatever, don't ride it with a broken seat tube. I would suggest you take a very close look at the rest of the frame and welds on that bike. It might be time to look for another one.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"