Hi Andrew - I have just about every other bike/wheel size, except a 650b bike. IMHO it's the same effort to build up any other bike. I have ****loads of parts laying around collecting dust that will work for this. Just need some 650b rims n spokes and I'm on my way. Both wheels will be 650b. I have a brand new 700c fork with disc mounts that may work. I will check the geometry and calculate what fork rake and trail should be. However, if I have to build a fork to fit and maintain good handling I can do that. It's more of an experiment I am willing to take on to see how it works out. :-)

From what I have read the existing canti boss placement in the rear will work with long reach brakes. Since I got this bike at a garage sale from a meth addict for free there was no fork, nor paint! IMHO there are plenty of options for a 650b bike. I went to NAHBS in Sacramento and there were quite a few 650b bikes on display.