1. Cut the top off
2. Drill out with appropriate size big ass drill bit
3. Using appropriate expanding reamers to finish it
Or let Joe Breeze do it...
I remember selling those back in the day. We sold every we could our hands on. Couldn't order them fast enough. In all my shop years it was the only bike we sold with premium.....maybe a $50 premium. Owner of the shop bought a 17? and never opened the box. It went in the basement with other special bikes/frames.