Any type of brazing to do this repair is not a good idea. TIG welding the steerer does work, with some skill and care. Be advised that Reynolds 531 tubing is not recommended for welding, 4130 CrMo is fine though. Here is what I have done for this repair. The cut should be in the area where tubing is still 22.2mm (0.875") and below the threads, Avoid making the joint right where the stem wedge or cone will be. Make the cut have a 45 deg bevel on the outside, on both the fork and the new tube. There should be no chrome or paint near the joint, and clean the inside of the tubes also. I use a solid steel mandrel, 7/8" cold finished 1018 is OK. This should fit inside the fork and extension tube nicely with just a little clearance. There should be zero at the bottom of the V groove, both tubes evenly cut at 90 deg. Tack the V groove joint in about 4 places, use care to not weld the mandrel in place! If you are a good welder, the mandrel can be left inside as you carefully finish the joint. If even a tiny bit of weld sticks to the mandrel you are in big trouble, check to make sure it's free after short welds. Rather than take the chance of welding the mandrel in place, you can choose to remove the mandrel after tacking. In this case it most likely will need a reamer to clean up the inside of the steerer. I have done this repair many times, and never had any issues. It may be difficult to get anyone to do this and risk the liability however.