Call D&R Marine
I'd start by giving Rudy Nickerson at D&R Marine a call, he is the expert O'Day Parts guy. If you have a local company that builds masts, they should have the dimensions to make the part. Rudy will know the whole story though, so I'd still check with him. I'll link his web-site below, or you could call him at 508-644-3001.I hope you can fix the mast, The O'Day 12 was a great little sailing dinghy! I had an O'Day Widgeon, same hull, different sailplan.